This is one topic that has been much too mysterious to me since starting to brew 10 months ago. I understand that different commercial packages have estimated numbers of yeast cells and that you can closely estimate how a starter increases that number. I've also come to find that buying a new yeast for every batch gets expensive. To that end, I've been either harvesting yeast form my favorite commercial brew or making a stepped started from a commercial packet and splitting it and keeping enough for my next batch. Because I'm not dealing with commercial packets, I'm really at a loss at how to estimate how many billions of cells I'm actually pitching. When I have a little yeast cake at the bottom of a pint jar, how in the world do figure out how to get to the right pitching rate?