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Well, you have to look at the overall meal, how many sides etc...fewer sides needs more meat etc...I would look at the fact that there are two meats, and figure 1/3 pound total cooked meat serving per person in a normal meal situation. This allows for men who eat more and women who eat less. If it is all men, go 1/2 pound cooked meat per person (shave the side portions, as men tend to eat meat and don't eat as much of the sides).
Tri-tip cooked yield loss is about 15% of weight to medium rare, 20% for medium.
Chicken is around 30% of weight to proper temperature.
Assume 50 adults by 1/3 pound serving, you need 17 pounds of meat total. Split the difference, you end up with 8.5 pounds of meat of each type.
8.5 pounds x 1.20=10 pounds of tr-tip raw
8.5 pounds x 1.30=11 pounds raw boneless chicken
Thus you need to buy at least 10 pounds of trimmed tri-tip and 11 pounds of boneless chicken. This would be for full sides or sandwiches. If you are doing a meat and two meal, with say beans and slaw, tack on another 4 to 5 pounds to the meat order.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less