I agree with HBMTN, a lot has to do with what you do well, what folks expect and how they eat. Last year, we did a troop feed and figured 1/4 pound pork and one chicken breast per person, and it was perfect. Along with sides, that was one very full plate, but, these folks could put away the food. For a more mixed crowd, you are going to have fewer big eaters. When you get up to three or four meats, most people I know figure down to 1/5 pound (3-4 ounces) of each meat. And they run out, somehow it has become both acceptable and some kinda of weird mark of honor to run out of BBQ meats.