1st time BBQ concessionaires- need help

I was wondering if there are any basic formulas for figuring out tentative projected sales at an event based on attendance??? This event is at a suburbian fourth of July event and has about 15,000 people each year. We have a good spot right by the live music. Any suggestions would be appreciated. Thanks!

Some questions about the event:
Is this an all-day event or just evening? (Lunch time, supper time, both, or neither?)
How many other food vendors will there be? (How many others will offer BBQ?)
Is this an event where an admission is charged at a gate, or can people wander in and out at will?
Will some families be bringing their own picnic baskets, or is no "outside" food allowed?

Who's your competition? By that I mean how many other vendors are there and what are they serving? Have you a cool looking joint? Remember flash equals cash! On big shows with a lot of same/same as well as lots of variety look for 1-6% but with huge attendence that's a lot for a grab joint. Smaller shows such as this, if you have no competition you could snag at up to 25%(1 in 4 is way tough to do). That's if it's an all day event and the weather is nice, product is good, your the only one bringing the meat to the party, you grab exposure and have a great location. Is there no one on an organizing commitee or board that can give you ball park figures from past events? They should have a pretty good idea especially if they charge your rent by percentage. They usually have a $ per cap of the last few years on paper somewhere. Tough to say with limited info. Fairs, festivals and special events are different. Lots of variables like can the families bring their own food in? Does most of the crowd come in the evening for the fireworks? Does the music go on after the fireworks? Is there going to be a midway? Are you on an independent midway? Does the local fire dept., K of C, Kiwana's and other non-profits set up as well(they eat up customers)? Is your product a higher end meal or simple and reasonably inexpensive? You'll serve no less than 500 I'm guessing at the very least. Keep tacking on points if the criteria is in your favor. Myself, I would plan on 1500 servings if I have a great location and have back up plan for more. 10% ain't unreasonable for small events but then I'm a pessimistic/optimist. I'll get out there and sell because I have no pride except when it comes to my food. That's another thing...Do they let you get out and sell if it's slow? Some don't, but I don't ask. Can't do it on carnival lot but can on some special events. I definitly would ask the higher powers for any and all numbers they can give as well as any other vendors they may be able to help you contact from the PAST. I've never done BBQ on a show but I have watched BBQ joints be the top grossers on many. I don't like setting up next to them (also ladies auxillery fried chicken dinners) unless I'm doing something sweet. They've killed me! I wish you so much luck! If you've never done this before you'll never have so much fun pulling your hair out. HAVE A BLAST !!!
I'd also suggest you might think about signage (nice!) and clam shells and bags for carryout. People will take BBQ home if they know they can.

The event used to be a one day event of 15,000 people. They expanded it to two days this year. It is noon to 10 on the 3rd of July and 10 to 10 on the 4th. We are the only ones selling BBQ, but I don't know for sure how many other vendors there are. People can come and go, and I think they can bring their own food since it is a big, local park. We are right next to the stage/live music. Entry fee was 100 bucks. The park fills up we are told. There are no pop/soda restrictions, so we don't think there are too many city vendors. Great idea about letting people know we will sell in bulk...

Actually the projected attendance is 20,000. Also a vintage car show in the middle of the afternoon.