The statistic of "0.5%" that Bettany Hughes gives in the interview you have quoted here is most likely a rhetorical one that she made up off the cuff in order to make a point that women's stories only make up an extremely tiny portion of recorded history. This statistic seems to be repeated in dozens of sources published after the interview you have referenced, but I cannot find any reference to it in any earlier source, nor can I find any source that uses any kind of quantitative analysis to justify it.

That being said, Hughes's overall point that women's voices have been systemically silenced from the historical record to a greater or lesser extent in cultures all around the world throughout history and that the vast majority of surviving historical sources are written by men about men is undoubtedly correct.

The asker of this question expresses skepticism regarding the minuteness of Hughes's percentage and suggests that the percentage is surely higher than what she says. It is therefore worth noting that the extent to which sources by or about women survive varies significantly from one region and time period to another. Looking back through the recent answers that the asker of this question has written in this subreddit, I see that most of them seem to be about modern North American history, a period for which sources by or about women are relatively plentiful.

Bettany Hughes, by contrast, specializes specifically in ancient Mediterranean history (especially the history of ancient Egypt, Greece, and Rome), which is one area of history in which information about women's lives and stories is particularly lacking. Female authors only made up an extremely tiny fraction of authors who existed in the ancient Mediterranean in the first place, works written by a woman are less likely to have been preserved through repeated copying than works written a man, and the male authors whose writings have survived rarely say much about the lives and stories of women.

Compared to historians who study the ancient Mediterranean, historians who study modern North American history have an enormous wealth of surviving sources about the lives and stories of women. I mean, there are well over a thousand more surviving complete poems by Emily Dickinson alone than there are surviving poems by all the female ancient Greek poets (i.e., Sappho of Eresos, Kleobouline of Lindos, Telesilla of Argos, Erinna, Anyte of Tegea, Nossis of Lokroi, Korinna of Tanagra, etc.) combined.

There's no "workings" to that statistic because it's not a statistic. It's a rhetorical device made for the sake of emphasizing the very real point that women are often marginalised by the entire process of 'history making'. So not just in terms of past (or present) societies marginalising the influence and relevance of women, but in the historiography itself. Who gets to write down history? What are their biases and prejudices? What is the purpose of their work? Because it almost certainly wasn't to provide an objective and impartial record of the truth.

This kind of historiographical rumination is very much in line with a feminist approach to history, and there are many valid questions raised by that school of thought. On this matter I can really only speak about European historical canon, but it is very much a male dominated one - going all the way back to the Classical Greeks, it's almost exclusively written by men, about men. I encourage you to try this thought exercise: how many famous women from this vast span of history do you know about? Now how does that compare to famous men? How are those women portrayed? The enormous disparity between the depth and breadth of records and agency given to men compared to women is glaringly obvious once you start looking for it, and that's the real point that Hughes is trying to convey. It's even more noticeable when you read chroniclers like Gregory of Tours that make a point of demonising independent and successful women as evidence of the corruption and moral decay of secular society, compared to the righteous and godly ways of the male-led church. Just because the struggles, motivations, and ambitions of women weren't documented in favourable terms by old/rich/Christian men, it doesn't mean they didn't exist.

A bit of tangent here, but a very noticeable exception to this is the Alexiad, written by Anna Konmene while she was exiled after a failed coup. It's a part history, part autobiography, part lamentation of the reign of her father the Byzantine Emperor Alexios I Komnenos. It's a very unique primary source as it is written by a female author and has a 'feminine' (but not feminist) angle on many matters normally not part of the realm of women due to her significant involvement in royal life, such as the logistics of military compaigns, the Crusades, religion, politics, and a female (but again not feminist) view of the role of women in society. It's a very, very valuable insight into that period of history - but one that only ever manifested due to Anna Konmene's extremely privileged and exceptional circumstances. The vast majority of women in history were never afforded that opportunity to record and be recorded, hence Hughes' assertions that women have been buried into the margins of history.