Luckily Google Books has that page on preview, but unfortunately it isn't elaborated so I will just sort of have to guess where the figure is coming from, and so my response is yes, it is technically correct, sort of, but the numbers game is a bit slippery.

This is really more of an issue with how bondage is being defined, as it is an issue that can be looked at in many different ways. In this particular case, I find it implausible that this figure could be arrived at without including much of the population of China. And indeed, from European eyes it is possible to make the argument that China's population was in bondage to the tyrannical power of the emperor (in this case the Manchu-origin Qing Dynasty) but--as you can probably guess--I think that just because we can doesn't mean we should. presumably, the idea that the mass of Chinese were in bondage rests on two pillars: officially speaking, everything was property of the emperor (technically true but practically speaking property rights existed), and in a practical sense the Chinese people were subject to corvee. Corvee can be considered bondage, except that it was considered a tax, and from that perspective, taxation can be considered a form of transferred corvee (given that taxation takes a portion of labor compensation--wage). My real issue with the notion of enslaved Chinese is that there actually various systems of unambiguous bondage in China, from serfdom to full slavery. It had been declining for some time (in the Song dynasty outright slavery was outright banned, to variable effect) but still existed. The average farmer, on the other hand, would be in a situation not unlike an English commoner on estate land, and it seems wrongheaded to lump them in with slaves.

Granted I am making a lot of assumptions about Forsythe's methodology here but in my defense I can't actually see his methodology. If someone has a better idea at how the number is arrived at I would love to know, but my immediate reaction is that the figure is perhaps technically correct but not meaningfully so.

Much of my information about Chinese labor bondage is from William's Rowe's superb Crimson Rain.

The line in question:

David P. Forsythe wrote: "The fact remained that at the beginning of the nineteenth century an estimated three-quarters of all people alive were trapped in bondage against their will either in some form of slavery or serfdom."

The quoted source on Wikipedia:

David P. Forsythe (2009). "Encyclopedia of Human Rights, Volume 1". Oxford University Press. p. 399. ISBN 0195334027

To some degree this question hinges on a definition of what free and unfree labor mean. For example, legal historian Robert Steinfeld has argued that "free" labor in the modern sense of the term did not really exist in the Anglo-American world before the second quarter of the 19th century.

"Free labor" implies a number of things: (1) that workers choose their jobs voluntarily, through entering into contracts, rather than through coercion, (2) that workers have the right to leave their jobs at any time if they are unhappy with the conditions of work, and (3) that workers' incentives should be financial (i.e., wages) rather than coercive (i.e., whippings and beatings).

Before the late 18th century in England and America, none of these conditions applied. Under the English Ordinance and Statute of Labourers of 1349-1351 servants were required to sign on with their masters for for a fixed period (usually one year), were subject to criminal punishments if they failed to complete their term of service, and could be required by law to enter into fixed-term contracts - i.e., otherwise they would be considered "masterless men" or vagrants, subject to imprisonment. The masters retained the legal right to beat their servants if they failed to live up to the terms of service.

The distinction between servitude and slavery in this context was not that servants had inalienable rights to their own person - they didn't - but that servants (unlike slaves) consented voluntarily to the terms of service.

This legal interpretation shifted in the early 19th century as American courts increasingly refused to invoke criminal sanctions or recognize the right of masters to beat or sue their employees for their failure to live up to the terms of service. So workers became "free" to unilaterally break their labor agreements with legal impunity. The employer could sue the employee for breaking a contract, but could not invoke criminal sanctions to force the worker to complete the agreement.

The result was a more expansive definition of "free labor": whereas servants and slaves in the 18th century were located at different points on a spectrum of unfreedom, by the 19th century there was an increasingly stark contrast between freedom and slavery when it came to the ability to command one's own labor.

Source: Robert J. Steinfeld, The Invention of Free Labor (1991)

Would he consider women in countries under English common law to be in bondage?

The thing is the idea that 'serfdom' and 'slavery' are distinct and useful concepts is really what does the TIL wrong. It is better to think of labor in different levels of coercion, some of which might be termed slavery or serfdom depending on degree. This avoids a lot of the stranger implications of this - ostensibly free labor in the south in the post-civil war era was held in conditions that were less free than supposed serfs/servants in imperial China. So defining this in terms of exact percentages falling in these groups is totally arbitrary. It might be better to observe that "three quarters of the world population was in some sort of coerced labor system during the start of the 19th century". This definition is so wide though as to make this statement somewhat silly. Everyone is coerced to labor a little bit - if you don't work, you don't get money to get things you want. That's coercion, though one that we generally find acceptable.