One of the mistakes I made when I first rinsed yeast was I didn't leave enough beer behind when racking to give a higher liquid to yeast/trub ratio. That really allows the trub to settle out much better allowing you to get more yeast. I want to say that the recommended ratio is one part yeast/trub to 3 parts liquid. For example, when I washed yeast last, I had a 4 pint jar. I racked my beer (getting as much beer as I could) then put 1 pint of the remaining yeast/trub into the jar. Then I topped the jar off with boiled cooled water. I let that sit in the fridge for a couple of days, poured off the liquid, added back boiled cooled water and shook it up. I did wait a little longer than 20 minutes, but by that time you could definitely see that a good amount of trub had settled. Then I just decanted the yeast on the top into a pint jar.
If you want to know how much yeast to estimate, there are pretty good guidelines at mrmalty.com. Here is the link to the calculator:
And here is a link that gives guidelines on how to use the "Repitching from Slurry" tab:
Basically, I think you can get what you want by assuming a higher non-yeast percentage.