I never cooked buffalo roasts, but If you are estimating the food being served on the raw product, you have not calculated for any loss due to cooking.
An average cooked serving of meat weighs 1/3 lb. The amount of raw meat required for that serving depends on how much the meat shrinks during cooking as well as the particular cut, its size, fat and bone content and the degree of doneness.
There are two kinds of cooking losses. Yields may be reduced by cooking/shrinkage losses and by wastage in carving and serving. The cooking or shrinkage loss is the actual weight difference between the uncooked cut and the cooked meat before it is carved.
Shrinkage occurs when water evaporates from the surface of the meat and when fat, water and juices drip from the meat. Shrinkage is affected by cooking method, duration and temperatures, and degree of doneness. Shrinkage during cooking is inevitable and it occurs with every cooking method. It can be as low as 10%, or as high as 50%.
Generally, average cooking osses range from 1/4 to 1/3 of the raw meat weight. Remember, cooking losses in small roasts and portion cuts tend to be greater than in larger cuts.
Most of the meat is comprised of water, and as it cooks the water evaportes. Depending on the desired doneness "rare" would be less likely to shrink than "well" would.
Remember, the temparture of the pit is important, the harsher the temparture, the more moisture will be lost and it will cause more shrinkage.
It is important to for the meat to rest before you cut it, this way the juices distribute back and it stay moist all the way through the meat.
Slicing and serving losses are due to fat trim, poor carving, or smaller portions that are not suitable for serving.
Humphrey's DownEast Beast W/BBQ Guru