Bob that does not support slurry...
For harvesting from post fermentation (primary/conical etc). I would use this https://www.wyeastlab.com/com-yeast-harvest.cfm
as a baseline.
For starter harvesting, the % of slurry that is yeast is much higher so I would use a 3.33x the density based on my own personal experience.
For me, I can get about 40B cells into a 40-45mL slurry after decanting the starter. Roughly 33% slurry, gives a density around 2.89B per mL.
From there you can calculate the total amount of cells you had when harvesting, apply the viability loss of
% remaining = 0.8^n where n is the number of months since harvesting.
Then you have the approximate number of cells remaining to date. Enter that into the yeast starter calculator of your choice.