The math behind the calculator is somewhat outlined on Jamil's site - what isn't there can be derived from working backwards. To my knowledge the actual math isn't published anywhere.
If you're really interested in getting into the nitty gritty of yeast and pitching rates then buying a microscope and hemocytometer is totally the way to go. See info on cell counts here:
http://www.whitelabs.com/beer/cell_count.htmlI got my supplies very cheaply from ebay:
microscope:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 456wt_1139cheap hemocytometer:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 902wt_1139There is plenty of info on cell counts in Chris White's and Jamil's yeast book.
You'll also need .4mm slip covers for the hemocytomter - the ones included are only .17mm which isn't enough to overcome the surface tension of a sample.
For a year or so leading up to actually purchasing the hemocytometer I used sedimented cell volumes in a 50mL container - and I actually still use this as a double check for my counts - it's been completely accurate so it's a viable option. Check out the Wyeast labs commercial site for more details on sedimented cell volume:
http://www.wyeastlab.com/com-yeast-harvest.cfm