I find the yeast picthing calculators handy and convenient. Gives you an idea how much yeast you need for every batch. http://www.mrmalty.com/calc/calc.html
I have counted counted cells with a microscope and hemocytometer and matched it very closely with these results.
I have also counted stained yeast cells using an hemocytometer and a microscope. It is an interesting exercise and necessary for commercial brewing, but has limited use at home. At this point, I really do not find yeast calculators to be all that useful. I find rules of thumb combined with my experience with a culture to be more useful. That being said, what is interesting is that Mr. Malty's standard re-pitch suggestion is 173ml, which is basically a 1L starter at maximum cell density. His ale pitching rate is 500ml at maximum cell density, which solidifies my assertion that underpitching by as much as 50% makes no difference in fermentation outcome while providing enough new cell growth that a culture can be serially re-pitched more than a couple of times without worrying about a huge loss in viability, that is, as long as one's wort is well aerated. I have always re-pitched between 150 and 175ml of settled slurry per 5.25 gallons. That is based in the rule of thumb that 1ml of settled, thick slurry contains approximately 1.2 billion cells as well as the accepted average maximum cell density of a 1L starter, which is 200 billion cells. If we divide 200 / 1.2, we get 167ml; therefore, pitching anywhere between 150 and 175ml of settled, thick slurry will yield approximately an equal number of cells as a 1L starter at high krausen. Pitching less less slurry will yield more new cell growth in well-aerated wort while pitching more slurry will increase the average age of the cells in the culture due to suppressed new cell growth. That is why overpitching is a poor practice in a serially re-pitched brewery. It is better to improve brewery hygiene than overpitch to avoid contamination from wild microflora. For most part, we do not base pitch rates based a fermentation reaching projected terminal gravity. That is function of genetics and dissolved O2
demand. We can seriously underpitch if there is enough dissolved O2
to support the number of replication cycles necessary to reach maximum cell density and our brewery hygiene is impeccable. Suggested pitching rates are for commercial brewing, which accounts for the difficulty encountered in keeping a commercial brewery and brewing equipment spotless. Pitching rates are primarily about outcompeting competitors.