Jamie - I work my gigs, the same as you do. But I also state in the contract the amount per person goes up (for the extra guests) by 50%, if the number of guests exceeds the agreed contract amount. I too cook extra, but that is to cover my arse and not look bad for running out of food. I have never ran out and if I did, it would be because the party invited extra guests without notifying me.
Diamond Plate FAT 50 Ultra
55-Gal Horizontal offset
2 55-Gal Vertical Barrels
Anything worth fighting for is worth fighting for dirty!