I have found through my experience that pitching at a rate of .75 million cells/mL/degree plato(+/-5%) for ales as prescribed by Jamil's calculator has rendered very good results, as opposed to underpitching at a rate of roughly half that (1 vial) for 5 gal of a 50GU beer. The end results speak for the themselves in this regard. This assumes a viable (healthy) yeast pitch.
To accurately count yeast cells, one would need a microscope and a hemocytometer. I am assuming the cell counts on the vial/smack packs are 100 billion cells of viable yeast. This is an assumption one must make when estimating and growing a yeast pitch.
My advice to anyone that's skeptical of Jamil's yeast pitching rate calculation is to do a side by side blind tasting, by splitting an appropriately sized batch of wort and pitching at .375 and .75 million cells/mL/degree plato accordingly. It's amazing the flavor difference. Try it for yourself.