Hi Allen,
When I figure butts for a crowd, I do fairly simple math but always over estimate my needs slightly. I always like to have a little in the bowl left over to avoid somone having to scrape out the last morsel.
Figure around a 50% meat yield per butt after cooking. This accounts for moisture loss plus fat and shoulder blade removal. So each 10 lb butt will yield 5 pounds of edible meat. If you cook three 10 pound butts, you will have 15 pounds of edible product. It will be generous for you to provide half of pound of BBQ for each guest on average. Huge eaters may eat .75 lbs while dainty eaters may go for .25 or less.
To be on the safe side, if you cook four 10 pounders, will will have a good bit left over. Better to have a bit for the freezer than not enough.
Your calculations are on target.
