What criteria/formula did you use to determine this? I would like to calculate that in the future for yeast of varying age.

I just used a few rules of thumb. The counts are merely approximations, but all one needs is an approximation when it comes to yeast due to exponential growth. I used Wyeast's cell count for thick slurry that contains 40 to 60 percent yeast solids, which is 1.2 billion cells per milliliter. The maximum cell density for a 1L flask is approximately 200 billions cells. I am assuming that the average commercial yeast culture has about 60 billion viable cells by the time it is pitched into a starter or wort. I took 60 divided by 1.2, which gave me 50ml. I then discounted the viability of the cropped yeast by 25%, yielding 50 / .75 = 66.67ml. I then made it is an easy to measure value of 65 ml. One can expect to grow between 1 billion and 1.5 billion yeast cells per gram of extract A 1L 10% w/v (1.040) starter contains 100 grams of extract; hence, it will support the growth of between 100 and 150 billion new cells given enough O

_{2} to support cellular health. That growth range yields a ratio of roughly 2:1 new cells to old cells.

Remember, the only way to get exact cell counts is to take a small volume of slurry or actively fermenting wort, dilute it to make counting easier, and count cells using a microscope and a hemocytometer. Even then, the cell count is still approximate due to the error encountered when taking a sample.