As others are saying, 50% raw weight is probably a good place to start; your efficiency will often be better (especially if you inject or braise in juices), but it's better to end up with extra than run out. With Boston butts I actually tend to run ~60%, but that's beside the point.
When you allow people to make their own sandwiches, they tend to take a bit more than 4 oz. I myself tend to really pile it on, to the point of being made fun of. "Hey Phil, you know you can get more than one sandwich, right?" Everybody's a comedian...
If you're uncertain about how much people are actually going to eat, throw more sides in. I know that a big pan of baked beans alone will cut the amount of PP I need down by 1/4. It's amazing. Also consider putting on a couple slabs of ribs or some brisket for the connoisseurs; it's usually well-appreciated and again saves some of the shoulder.
In the end, don't stress. Just have fun! That's what Q is all about. You'll be the man of the hour in any case.