Part of your calculation should take into account how many people can live in an area and NOT be farmers.

I used some of my own research (as well as the Medieval Demographics link above) to come up with this base line set of numbers:
Simple Farming statistics

An average production to support a population of non-farmers works out to the following. With one square mile to work (640 acres) a community of 35 people can provide enough surpluses to allow 4 non-farm workers to live there. Each person farming (ages <7 is 0 worker, 7-11 is ˝ worker, 12> is 1 worker) contributes 2.8% to the yield.

Here is the link to my site with some of the background numbers and further information to do quick and dirty population summaries.

http://www.dragonslayers-society.org...ureAmpHoldings

BOB