Hi Scotty and welcome!
Here is a formula................
RW = (SP X P)/SH
RW = Raw Weight in lbs.
SP= serving portion in lbs.
P = number of people
SH = shrinkage factor in %
As Jim has mentioned, when it comes to serving portions....1/4 lb. is plenty for your average group. But, plan 1/3 lb. if all guys.
I always plan on 50% shrinkage for my butts.
RW = (0.25 X 20) / .50
RW = 5 / 0.50
RW = 10 lbs.