Last night, I attended a seminar in Indy presented by one of the Omega Yeast founders, Lance Shaner. There was one aspect of the presentation that caught my attention. He contends that its not truly applicable to target Cell Count as our metric for yeast starters. He said that the diameters of various yeast variants differ substantially and the amount of mass that those cells contain, also varies. For example, he said that Weizen yeast has much larger diameter and cell mass than something like a Saison yeast.
He went on to state that yeast growth calculators aren't necessarily accurate at predicting starter cell counts due to the differing individual cell mass. However, he did say that their research shows that for a given starter size and gravity, the total biomass produced is relatively consistent...regardless of the cell mass differences between yeast variants. So you might get a far lower cell count when growing a Weizen starter compared to the same size Saison starter, but the sum of yeast mass in each starter is going to be quite similar.
That was surprising to me, but its not when you figure that the yeast are probably utilizing the wort and growing biomass at the same rate, but putting it in different places. One makes fewer, but bigger cells and another makes more numerous, but smaller cells. He did say that pitching rate calculators are flawed do to these factors, they are still reasonably valid for sizing starters.
The final aspect that surprised me was that it is the amount of yeast biomass added to your batch that matters, not cell count. That also translates to a volume of high-viability yeast slurry that should be added to your batch. When he was pinned down, Lance did say that a typical gravity 5-gallon batch would need about 50 mL of high-viability yeast slurry. As I recall, the old clear White Labs yeast tubes probably held about that much slurry. That sounds like an easy to remember guideline for how much yeast slurry we should really be pitching into our batches if you're repitching slurry. Overpitching or pitching directly on a yeast cake may not be best for fermentation.
Give this some thought!