I don't even bother with proper calculations. Most beers I make "require" around 250-300 billion cells. The chart from Yeast: The Practical Guide to Beer Fermentation, states that the greatest growth from one vial/pack of yeast, is a 1.6 liter starter on a stirplate, and it should produce around 300-350 cells depending on the age of the yeast (as far as I understand it).
From what I've heard and read, you really can't overpitch a 5 gallon batch with a normal sized starter, not even if you pour a new batch on top of a yeast case from a previous fermentation (which has way more cells than a starter).
So for every starter (for a 5 gallon batch) I make, I boil 1.7 liter of water with 160 grams of DME. After 7-8 minutes, I'll have 1.6 liter of wort with a gravity around 1.037. Once it has fermented out, I cold crash it and decant the wort before pitching. So far, every batch has taken off within a few hours without any ill effects of under/over pitching.
Simple to remember and works every time.