There are plenty of calculators that tell you how much starter to make for a specific amount of starting yeast and for a specific gravity of the wort. On packages/vials of yeast it tells you the estimate of how many yeast cells there are in there, however when you harvest your own yeast how are you to know how many yeast cells you have? How do you know how much starter to make if you don't know how many yeast cells you are starting with?
The "scientific" way would be to use a microscope and hemacytometer to count cells. The empirical method is what I use and has worked well for me for several hundred batches. You make a guess! I use between 1/3-1/2 of a previous slurry if I'm going to direct pitch the slurry. I use between 2 TBSP. -1/4 cup if I'm going to make a new starter from slurry.