100 Cornish X's, On average how many should i expect to lose?

25% crop loss is not unheard of, so you are doing well from that perspective.

 

Your doing very well, but it may be time to consider restricting their feed. Your ready to butcher in a week or 2 so feed them only what they will eat overnight/morning and in the heat of the day let them just have cool water. Digestion creates lots of heat and they don't tolorate heat well so that can remove some stress, as well they tend to over eat at that age anyway. Give them more space at the feeders and just more total space to be sure that trampling and comitition are not factors in some not doing well.

If your losses continue at that rate start your processing with the big ones now and continue till you have those that look like they are not at all athletic done.