Originally Posted by Gameface
How is viability determined? I imagine viability is highly dependent on the very specific circumstances that pack of yeast has gone through. If one shipped via ground from the West Coast to the East in August, then sat in a storage room for days before being placed in a fridge, only to be purchased by someone on their lunch break who left the pack in their glove box, well they are going to have fewer cells than a guy who lives up the street from Wyeast who picked up his pack the day after it was packaged and put it right into his fridge within minutes of purchase. Even if they both use the yeast the same number of days after packaging one guy will have less. That throws all the rest of the numbers off.
They're calculators, not crystal balls.
Yeast calculators like Beersmith, Mr Malty, and YeastCalc are designed to give you a rough estimate of what your yeast will do under "optimal" conditions. There are far too many variables involved in the propagation of yeast, for a mathematical model to produce precise numbers.
However, that does not mean that they are not useful tools.
At the very least they give the homebrewer a consistent, repeatable procedure for making starters, by which some of the variables can be reduced. The key concept here being 'consistency', something that all homebrewer's should strive for in their brewing process.
At best, they are fairly accurate, if the yeast has been handled correctly.