I'm going to be re-pitching from slurry for my next batch. The MrMalty calculator says I need ~400B cells. The beer I'm harvesting from has been in primary, on the yeast for 55days. I'm going to pitch the slurry to the new wort the same day I 'harvest' the yeast. So the calculator says I need 200mL of slurry to get the 400B cells. But I'm just thinking, this yeast has been sitting at the bottom of primary for a long time....is that harvest date really accurate for predicting the viability? If I just play around and say it was harvested 25 days ago, then I would need twice as much slurry.
So, I'm just trying to figure out, is the viability of the yeast slurry a function of when I harvest it? OR is it more a function of when it finished fermenting and settled out? They yield significantly different answers for required mL of slurry for re-pitching.