The whole yeast cake will contain about 4X the yeast required to brew a batch of the same strength as the original beer.
You can ratio that amount directly to the gravity of the beer you are about to pitch to. Say you harvested from a beer with an OG of 1.040, and are pitching to 1.080, then you will need twice the yeast (or half the original cake). If you go from a 1.050 to a 1.075 you will need 1.5 times the yeast (or 3/8ths of the cake).
It's a very crude method, but it beats figuring out what quantity of the slurry is yeast. And this works for a thick or thin slurry, or for one with lots of hops and trub versus a clean one.