I was catching up on Noahpinion, and a post on Myths of Ancient China caught my eye. I strongly suggest reading it (and the comments, which contain some excellent discussion and data). First of all, I agree with the idea that long run comparisons of China with the rest of the world can be misleading, data is too uncertain and the physical landscape has changed too much.
What I'd like to add is that I strongly agree that there are flaws with a lot of the per capita GDP estimates from previous eras when used in current arguments. If your interest is solely in what the actual per capita GDP was, agricultural output is probably fairly accurate.
However, if your interest is in making a statement about the modern world, then the measure looks less adequate. Just about every history book that I've read on the subject mentions that according to both contemporary accounts and archaeological findings, there is a large variance in the material prosperity of individuals at different places in times. This and not the relatively greater contribution of agricultural resources to real per capita GDP is generally what is relevant to modern discussions which focus on access to material resources.
Agricultural resources in the pre-modern world largely fluctuated with availability of land, focusing on this gets really odd results of living standards rising when population is falling and living standards falling when population is rising and contemporaries are writing of great wealth and affluence. If the purpose is to draw comparisons about the economic system as we understand it than it is necessary to attempt to distinguish changing levels of access to goods from access to foodstuffs since these seemed to vary largely independently. Though to the person faced with a shortage, having better tools, more knick knacks in the home, and a greater variety of occupations available is likely little comfort for an empty belly. If however, the question is in regards to technological change, economic sophistication, market integration, and changing material wealth Maddison's approach is likely insufficient.
Since I've seen the supposed stagnation of pre-modern economies brought up on occasion I'm mostly just really happy to have a post by someone other than myself that addresses ancient Chinese economic history. I think it is a critically important subject to understanding actual economic changes to provide a contrast with western development, which is the used too uncritically as a sole reference point.
Now to find some blog posts on middle-eastern economic history...