In his book, Chris White said that the vials held 47 mL with an average fill level of 36 mL, and that when the yeast settled to the bottom it occupied a space of about 14 mL. Fully dispersed, cell density would be about 3B/mL, and packed, 8B/mL, he said. So volume of slurry is going to be a fairly arbitrary way of estimating cell count or mass, to whatever extent that is really needed, as estimating the density by eyeball is going to be something of a guess. If you develop a consistent, repeatable procedure that works for you, quantifying it is unnecessary. Results are the best guide.