I don't normally cook pulled pork for large groups (40 has been my largest to date) but when I do I usually count on ½ lb of raw butt per person. I notice you use ¼ lb of raw butt per person. Can you elaborate? Is it because of all the sides that were provided?
I bought 20 lbs for my last cook for 40 and my DIL said they wiped the pan clean. I was thinking next year to up the tonnage. 
Cliff, I'm confused on your math as well.
Trust me; for large groups, the math works!
If 40 people ate 20 lbs of pulled pork then sides were either scant or there were a lot of folks who had to be carted to their cars in wheel barrows afterwards!
If there's one thing I've learned from the 30 or so large group cooks I've done it's this: 1/2 lb of meat per person is simply TOO MUCH!
► Sides play a HUGE role. Buns and sides, especially starchy sides (potatoes, Mac-N-Cheese, etc), are very filling.
► You always get a few who will eat more, but you also get some who eat less and in a large group you are bound to have at least a few who won't eat any meat at all.
In fact that math works pretty well for sides too. But if you plan to cook extra of something make it the sides, because they cost less.
Try this: go to Burger King and order a Double Whopper (1/2 pound meat) with only one side (fries) and a drink. See how full you are after eating it. I am miserable if I eat that much.
