Just found this from Stogie:
"Here is the formula....
50% yield from the butt, so 15 lbs raw weight will get you 7.5 lbs. of cooked meat. Plan on 1/4 lb. sammiches so you get 30 sammiches from that 7.5 lbs. Of course you can plan on 1/3 lb. sammiches but sounds like you got lots of sides."...