I give you linkage! Phenomenal tools, all of them. If you know the space, the climate, and how long it's been inhabited, you get a "current" population you can play with as you please.
Medieval Demographics Made Easy
The Domesday Book - Medieval Demographics Made Easy
The Welsh Piper » Medieval Demographics Online
Those are great tips/resources, guys!
If you enjoy the mathematical modeling side of things, which all of those numbers up there suggest you might, you may also want to peruse the Population Dynamics pages at Wolfram.
For a nice primer on natural logarithms, I recommend: An Intuitive Guide To Exponential Functions & e | BetterExplained