I think you need to consider the context of the disagreement between Lemon and Hoff Sommers to really make sense of the issue. To start, neither of these two women are historians - Lemon is a professor of law at Berkely, where she's a widely respected authority on Domestic Violence law, while Hoff Sommers is a philosophy phd whose main activity these days is writing right wing political tracts for the American Enterprise Institute. So strictly speaking, the argument they're having about the "rule of thumb" isn't a historical one - rather, what you're looking at is a feminist legal scholar and an anti-feminist ideologue who are arguing over the existence of the concept of the rule of thumb. And on that point - whatever historical errors might have crept into Lemon's argument - I think Sommers largely failed in her bid to discredit Lemon's scholarship.

Let's start by looking at the claim that Hoff Sommer's originally made: the controversy starts with an article she wrote in the Chronicle of Higher Education, called "Persistent Myths in Feminist Scholarship," in which Hoff Sommers quotes the claim in Lemon's book:

"The history of women's abuse began over 2,700 years ago in the year 753 BC. It was during the reign of Romulus of Rome that wife abuse was accepted and condoned under the Laws of Chastisement. ... The laws permitted a man to beat his wife with a rod or switch so long as its circumference was no greater than the girth of the base of the man's right thumb. The law became commonly know as 'The Rule of Thumb.' These laws established a tradition which was perpetuated in English Common Law in most of Europe."

and makes the argument that:

The phrase "rule of thumb" did not originate with any law about wife beating, nor has anyone ever been able to locate any such law. It is now widely regarded as a myth, even among feminist professors.

Let's start by clearly the air a bit: Yes, Lemon's book was incorrect to suggest that Romulus was anything more than a mythical figure, or that we can trace the origin of the alleged "rule of thumb" to ancient rome. The problem is that Sommers used this error to try to discredit Lemon's entire argument, using it to go on a bit of a rant (seriously, read the link) about feminist legal scholarship, about feminism more generally, and to launch personal attacks against Lemon - where in reality the ancient roman connection is kind of incidental to the book's larger point, which is that for much of history, there really was a legal doctrine supporting husbands' right to "moderately" punish or beat their wives. This doctrine wasn't always supported by judicial decisions or by legislation, but there's a great deal of evidence suggesting that it was customary and widespread - and the key point here is that we know it existed - something which Sommers appears to be trying to deny.

When we look at the actual evidence, we find that the idea of something like a "rule of thumb" is actually persistently (though often obliquely) brought up again and again in legal history.

Blackstone's Commentaries on the Laws of England (mid-1700s) state that "by the old law"

The Husband might give his wife moderate correction. For, as he is to answer for her misbehavior, the law thought it reasonable to intrust him with this power of restraining her, by domestic chastisement, in the same moderation that a man is allowed to correct his apprentices or children... but this power of correction was confined within reasonable bounds.

Pointedly, Blackstone also asserts that

With us, in the politer reign of Charles the Second, this power of correction began to be doubted, and a wife may now have security of the peace against her husband.

But the point here is that Blackstone was acknowledging the existence of a doctrine of "moderate correction" under the old laws. And he also qualifies this by noting that:

Yet the lower rank of people, who were always fond of the old common law, still claim and exert their ancient privilege; and the courts of law wills till permit a husband to restrain a wife of her liberty, in case of any gross misbehavior.

So while Blackstone appears to be denying that something like the "rule of thumb" is actually "on the books," he's also explicitly acknowledging that it was on the books in earlier years, and that many people still regard it as legitimate.

Bacon's "New Abridgment of the Laws" (1736) actually suggests that something resembling the existence of a doctrine of moderate correction is law;

The Husband hath by law Power and Dominion over his Wife, and may keep her by Force within the Bounds of Duty, and may beat her, but not in violent or cruel manner.

Note that later editions and copies of Bacon disagree over whether or not this included husbands' right to violently beat their wives, or whether this doctrine applied in the US or England - some reprints and editors accept it, others don't. The point is that it's being debated - which means that at least some judges and individuals were at least trying to apply it.

Legal scholar Henry A. Kelly has analyzed the evidence for a "rule of thumb" at length, and while he concludes that there was no "actual" law or rule about the rule of thumb, he also finds a great deal of evidence that the idea of it was widely and continually brought up in legal cases and legal literature, both in England and in the Americas.

Kelly found a decision in England from 1778, for example, in which a judge told the court that "a husband could thrash his wife with impunity provided that the stick was no bigger than his thumb." And he does not question that "the doctrine that moderate chastisement means moderate beating" surfaces frequently in english and american legal history. Take, for example, his analysis of nineteenth century American court cases, in which he suggests that judges who made statements about how the notion that "a rod which may be drawn through the wedding ring is not now deemed necessary to teach the wife her duty and subjection to the husband" is actually pretty clear evidence that "there was a belief that husbands had a right to use a small switch on their wives," and that "there was such a belief about the wedding ring being the measure of a rod that could by custom be used against her."

The point here is that while Lemon is clearly incorrect about Romulus, and while Sommers' assertion that there's no "actual law" about the rule of thumb is essentially correct, she's committing a pretty egregious academic sin herself in trying to use this as a "gotcha" to discredit Lemon's scholarship, and feminist scholarship more generally. The historical evidence suggests that something resembling the rule of thumb was a pretty constant presence in Anglo-American culture - and the fact that we can find scholars and judges arguing about it or referencing it is a sign that this isn't something which modern-day feminists have made up, which is what Sommers seems to want to argue.

As far as the "rule of thumb" is concerned, I think Tom Tyler puts it quite neatly:

irrespective of whether this practice was officially sanctioned by law, it is clear there certainly existed a popular belief that, within these "reasonable" limits, wife-beating was tolerable. Thus, although the origin of the perfectly innocent phrase "rule of thumb" does not lie with violent abuse, it seems that the thumb was perceived as a de facto standard in this regard.

So in answer to your question, I would just say that as someone who is interested in history and is getting into legal scholarship, you should be prepared for this kind of "bad" history to surface a lot - judges and legal scholars are not historians, and they get stuff "wrong" about history constantly. Obviously, the "history" part of Lemon's book is not its primary focus or its strong point - but Hoff Sommers is basically grandstanding by trying to take one or two historical inaccuracies and using them to discredit what is essentially a legal argument - and one which is on much less shaky ground that Sommers' out-of-context quotation of a single error suggests.

If anyone's interested, I'd suggest looking up the exchange between these two on the Chronicle of Higher Education website - Lemon cites numerous sources to back up what her book says about the rule of thumb, while Sommers' rejoinder is titled (I kid you not) "Essentially everything in Professor Lemon's response is wrong." Read them both and you'll see pretty quickly that whatever mistakes might have made their way into their work, one of these people is following the conventions of academic scholarship, and making a well-intentioned, reasonable attempt to back up their main argument, while the other one is... not. I don't want to get political here but I think Claire Potter's response was bang on:

if lawmakers and jurists subsequently believed that there was such a person as Romulus, and based their judgments on documents that purported to describe ancient law and practice, one might still usefully refer to such a thing as the "Romulan rule of thumb" playing a role in the legal oppression of women. Take a look at Warren Burger's selective gloss on the history of sex in his concurrence with the majority decision in Bowers v. Hardwick (1986) if you don't believe that bad history can have a major impact on the law. Or arguments that rely on an entirely invented world history of race in United States v. Thind (1923).

tl/dr: Lemon is obviously wrong about Romulus, and she may over-state the level of legal/historical consensus about the "rule of thumb" - but Sommers is totally off-base in her attempt to use this to try to discredit Lemon's scholarship, and (lets face it) feminist scholarship as a whole, and she fails to actually grasp or engage with the larger point which Lemon was trying to make, which is that the custom/doctrine that men have a right to punish their wives has a long history. Bearing in mind that Lemon is a legal theorist rather than a historian, this isn't exactly a "gotcha," and errors like this are extremely common in legal literature - mainly because legal theorists are interested in the law, not history. Sommers' conduct and tone in the whole debate are completely beyond the pale, and (folks should be aware) that she's not regarded as a serious or credible scholar by most academics.