If I'm creating a yeast starter from previously harvested yeast I'm supposed to know how many cells are in my slurry to effectively calculate how much yeast to use in my next batch.
To me common sense suggests that a fully-settled layer in the refrigerator will be pretty consistent in the number of yeast cell per unit volume - some may not be viable, yes, but a consistent density non-the-less.
However, as far as I understand it, and according to Wyeast (http://www.wyeastlab.com/com-yeast-harvest.cfm) a yeast slurry is not the fully-settled layer but the yeast in suspension prior to it settling out (settling out is simply a mechanism to allow the brewer to gauge how much yeast is in a slurry). Therefore, isn't this purely down to how much water I've added whilst harvesting? How do I know how dense the yeast population is in a watery slurry like that? Isn't it best to work from the settled layer?
By my reckoning, and reading the Wyeast page, there are approximately 2 billion yeast cells per millilitre of fully-settled yeast cake. Is this not a more sensible way to gauge your slurry?