I would suggest using a yeast calculator because a repeatable process is the first step to learning what works. Then you can adjust your SWAG from there. It's better than dropping the bomb on your beer and hoping for the best.
Bingo. It's very much like using an IBU calculator. You're not likely to hit the measured number. You're probably not even particularly close. But you need some sort of benchmark for measuring your initial conditions, and some way to quantify what changes you make for subsequent batches. The calculator doesn't need to give you a number that equates to anything in the real world. It just needs to model conditions close enough where you can compare and adjust between batches.
And at that point the pencil and paper become the more important tools. In the end, we brew for ourselves and our own palates. A number like 55IBU or 250 billion cells means nothing until you can quantify the results with your own palate.