Yeast Caluculation Quick and Dirty?


For calculating starter:
So, am I thinking straight? It seems to me that for a 5.25 batch of beer you are looking at plato by a factor of 2 (give or take) and then put your decimal point where it belongs? If you figure a million cells per plato per milliliter times 20,000 milliliters to the batch size (rounded up) it would seem about right. By that math a 12 plato is about 240 billion cells needed per batch?
I realize some people assume more and some people assume less on the cells per milliliter.
And is it right that you take the last two digits of target gravity and divide by four to get plato? So 1.048 = 12 plato and 1.052 = 13 plato and so on?
For a simplification it could be thought of as a 24 rule then, i.e., double plato for the cell count and divide gravity by four to get plato. I realize the simplicity only works if you're doing a standard 5.25 batch size.
I'm not great at math, so if I'm not understanding please let me know.
Reason: correction
