One of the yeast calculators (I forget which one) gives a number of 1 billion cells per ml of fresh, densely settled slurry. I've always used that figure, and it has worked well for me.
I think that's about what I'm operating under as well. Without counting there's no way of knowing. So I just made a couple of "standard-ish" numbers to assume with all my slurry and stuck it to my fridge so I'll remember. I guessed pretty low on viable yeast and pretty high on non-yeast material just to be safe. That's where the "nuclear weapon" analogy comes into play.
Dumping slurry from a fermenter, unless it has been cold crashed and siphoned dry, usually ends up with enough beer of its own to split into about half slurry and half beer in the jar, so I don't need to add any to cover the slurry.
I use 8oz jars and assume that I've got about 50 billion cells in each jar if it's harvested slurry (jars not full, and no washing, so there's a good amount of non-cell material in there). If I overbuild a starter and save that, I assume I've got more like 100 billion cells (about 4oz of dense slurry). That's about 1 billion per mL, which fits what I've seen referenced other places.
I just started harvesting yeast, so I don't have a ton of experience, but my goal is to build a starter if it's past 1 month old, or if I am doing a full 10-gallon batch and don't want to use all of my harvested slurry jars up at once. Maybe I'll do a side-by-side of a starter and a straight pitch at 2 months to see how they compare.