75 people with an appx 5oz. sandwich. The last 3 times I have cooked PP for the Vol Fire, bone in with gunk taken out, we were at 55%.
75 x 5oz = 375oz of cooked pork needed. 375oz is 23.43lbs. 23.43 x 155% is 36.32lbs of raw meat!
So you are on the high side, but shoulder is cheap, and well, lets be honest, it is your BBQ, so the crowd is probably gonna eat 15% more then a normal BBQ joint!