Please throw the yeast cell calculators out with the trash, as they are next to worthless. There is absolutely no way to calculate the viability of a yeast culture and anyone who tells you that you can does not know what he/she is talking about. Yeast cultures are a heck of a lot more robust than most brewers believe. My whole beef with brewing software is that it attempts to achieve a level of precision that does represent reality. The only way to determine viability is to count viable cells using a microscope and a hemocytometer.
In practice, it is never a good idea to pitch a liquid culture that is more than a week or two old without first making a starter. Making a starter is not so much about increasing cell count, as it is about increasing cell health. The yeast biomass grows at a rate of 2^n, where the symbol “^” denotes raised to power of; therefore, it does not take very long for for a small number of yeast cells to become a very large number of yeast cells. What is important is sanitation because bacteria also grows exponentially and its replication period is 1/3rd that of yeast; therefore, the bacteria biomass grows by a factor of 8 every time the yeast cell count doubles.