The spread sheet is a great idea, and I am not trying to put any one down, but it has some issues, I am doing chicken for a friends party this weekend and was checking numbers on just the chicken. If you adjust the % of yield per pound of uncooked it does not change the # of chickens needed. It may very well be a good starting point but be careful. All that it figures in for the chicken needed is the amount of people to feed divided by 4 rounded up to the nearest whole number. It is not figuring in the lbs per chicken average, the yield per pound of uncooked chicken or even the amount per person per plate.
It may very well be a starting point to go off but double check the math.
And again not trying to put any one down or off and I am more than willing to help correct any issues, but do want to point out the issue that I have found in just looking at 1 item.