I think ravs is referring to the one based on Ghalev's "Medieval Demographics Made Easy." The version of the calculator I've been using is on welshpiper.com. It's at the top of their all time most popular articles.

The original article with links to a few different spreadsheets is http://www.io.com/~sjohn/demog.htm

It's an excellent article, researched from scholarly sources, and he's also peddling a book entitled Fief, which is equally good.