Food cost $
Shopping & prep time $
Travel expenses $
Sales tax %
Add it up and give them a bottom line, let the client do the division for a pp price
Base your numbers on a 75 minimum, and get your deposit
Request final head count 14 days prior as well as final payment, not your problem if it rains and only 50 show, never put yourself in a position where you have to cut a refund check unless its for a full cancellation.
If they cancel prior to food purchase take 30% of the balance due and refund the remainder, put this in your contract so everyone is on the same page.
If you already bought food, take your 30% and give them the food.
I'm not suggesting that anyone has to do business this way, this is just an example of how I do it, works for me.