Assuming you had a reasonable pitch at the beginning of fermentation, the complete yeast cake at the end of fermentation will contain roughly 4X the recommended amount for correct pitching of a beer of similar size and OG.
By reasonable pitch, I mean anything from just a healthy vial (about half the correct pitch rate), to a huge starter with 2X the correct pitching rate. The higher the initial pitch, the less the yeast reproduce and vice-versa, so the end result will be a similar amount of yeast in the cake.
So if you are re-using the cake within a few days, just take a quarter of the cake and re-use. No starter, just pitch. This method works whether the original beer started with a clean beer, or one with lots of trub, and whether you wash your yeast or not. And there is no need for you to estimate how much of the slurry is actually yeast.
If you store the cake for a month, a lot of the yeast cells will have died. I usually estimate a 25% loss, meaning that if I store the cake (in fridge in sanitized Mason Jars), and re-use after a month, I up the pitch to about a third of the harvested cake.