Confused by harvesting yeast

I plan to start harvesting my yeast out of my primaries soon, and this whole process of knowing how much you have and how much you need for your next batch confuses the crap out of me. Alot of programs tell you to use so many billion cells? Do the original yeast vials say that on them? My lhbs says they dont. (i have not used liquid yeast yet). So how would i know how many cells i have originally and how would i know how many cells i have after harvesting it?

This may help:

http://www.wyeastlab.com/com-yeast-harvest.cfm

http://www.whitelabs.com/beer/homebrew_information.html

White Labs info, I would assume you can find similar info on Wyeast. To answer your question about how to tell cell count...you basically make an educated guess. I would recommend reading the Yeast book by chris white and jamil zainasheff if you really want to get into it. A lot of in depth yeast culturing info that I just don't remember off the top of my head. I keep meaning to get into harvesting, but just haven't had the time.

The mr. malty calculator has an option for yeast slurry where it guesses how many cells you have based on time since harvested.