Kinda depends on the demographics of your expected crowd. Heavily male?  50/50 men and women?  Men will definitely consume the 1/2 pound you project, women in general won't (unless our 5yr old granddaughter is there....she'll out-eat the men on brisket).  I think 1/2 a pound is a reasonable guesstimate if you're feeding mainly men. Less if there are a lot of women involved.