There are basically two categories of selling BBQ. Vending and Catering.
Vending can be done as "road side", event, etc. It is basically you going out and "offering" a food product to the masses and hopefully finding people who will accept your offer in exchange for you "fee/price". i.e. the "contract" if formed at the point of delivery of goods. This type of food sales requires one to "guess" at the amount to prepare. This "guess" can be based on past event history, past actual sales, etc.
Catering is an entirely different animal. Catering is where a person (customer) comes to you and asks that you provide a service to them. Basically the "contract" occurs up front. It is entered into before any product is chosen or prepared. You present your product/pricing and the customer then tells you what and how much they are contracting for.
IMO your event is truly a catered one. If you attempt to treat it as a vend you will jeopardize your profit. It is my belief that you should have the customer decide on product & quantities and sign a detailed contract with a non-refundable 50% deposit. You CAN give the customer "advice", based on your knowledge, of how much food an average person consumes, etc. But let them make the final decision. Otherwise you will end up with waste/want and it will all be "your fault".
In my experience I would suggest to the customer 3 oz of meat, two bones & a 50/50 mix of white/dark chicken. Tell them that very few people will take all three of these items so they would have a built in buffer. BUT have them make the final decision. Remember, this is NOT a vend. You are being contracted to supply what they order.
caveat: this is how we cater but is absolutely not the only way to do it, but we believe it is the most profitable and offers the most satisfying outcome for both parties involved. :-D
