This thread has been far too quiet recently, so I thought I would add a comment or two.
A while back, I spent some time figuring how to tie together A Magical Medieval Society: Western Europe with the Domain rules in Birthright (and modified here). The resulting PDF was posted
here on the Expeditious Retreat Press' Community Support section. It is a 7 page PDF, so I am not going to repeat it all here.
The document talks about assigning the Population Density and Urban Population Rate figures to an area. I based a lot of this on the Provinces of Birthright. From here on, PD means Population Density, and UPR means Urban Population Rate.
The standard Population Density range is 30 to 160 persons per square mile for an average kingdom, out of a greater "theoretical" range of 0 to 200 persons per square mile.
The easiest correlation is to say that a Birthright province level correlates to a Population Density increase of 13, using the average scale. At that rate, Provinces rated 0 have a PD of 30, and are considered to be as sparsely settled as an owned province can be. A Province 10 correlates to a PD of 160, among the highest achieved historically. The PD formula, in this case, is 30 + (13 x Province Level).
The other option is to go with the broader, more absolute range of 0 to 200. In this case, each level should increase the PD by 19, and the lowest organized province (0) should represent a PD of about 10. The formula in that case is 10 + (19 x Province Level), for a range of 10 to 200.
The PDF discusses a more complex approach. First, climate and terrain act to boost or lower the Population Density. This effect varies by race. The PDF lays out some basic numbers suitable for the "standard D&D" world.
Next, the PDF talks about other terrain factors... the presence of rivers and waterways, and how they influence the PD and the UPR.
Moving on, discussion turns to the stability of the area. A border region will have a lower PD and UPR because it is less secure. A region deeper in the "interior" of a nation will have more security, and thus a higher PD and UPR.
The age of the region, in terms of how long since it last changed ownership, is a factor as well. Natural growth is reflected as a +1 to PD per generation of the primary race.
Technology level and the availability of magic are also factors that are given some weight.
Historical factors, like recent wars and plagues, are reflected as modifiers to the PD. These are considered temporary, and the penalties decrease by 1 step per generation. The length of a generation is an arbitrary estimate (those interested in something more precise are directed to the excellent article "Survival is a Group Effort" by Stephen Inniss in the Dragon Magazine Archive).
Now, as to how this ties in here...
If you know what the province level is, you can work backwards to reverse-engineer the detailed numbers. If the PD correlates to a higher province level than you have in place, that might indicate that you need to add some historical events ... war or plague... to explain the lower-than-expected population. Or, it may mean that the area is magic poor. These may spark some ideas for adventures to place there.
So, there it is. Have fun with it.
Oh, and if you are using these rules, post a message sometime and let me know how its going.