I work at a 2 station fire dept with 13 on duty. This is where I have gotten my practice.

For pulled/choped shoulder.

.75(pound)x(number of people)+1pound for every 5 people. If serving on a bun with slaw and bbq beans.

so for my shift 13x.75=9.75+2.6=12.35 or 12.5(for easy purchases) pounds before cooking.

for 100 people it would be 95 pounds.

I have done this many times and it works! I even did a few parties and the formula worked well.

each time there was enough left over for a sandwhich or two the next day.

Brisket, is just flat 1 pound per person, after trimming and cooking it comes out to .5-.75 pounds per person. Again these figures are for meat if other stuff is getting served too.