It would help if you found out what else will be served at the party and how it will be served. IF the pulled pork is the main attraction/only entrée, and if it's going to be self serve, Chef Willie's 1/2 pound per person number is spot on. If there will be other entrees and if someone is serving it up and controlling portions, then you might be safe with a 1/3 pound per person.
Also, boston butts have about a 50-60% yield. I usually go conservative with estimations and use the 50% number. This means that if you want 1/2 of cooked pulled pork per person, you would need 1 lb of raw butt per person.