How many attend the event?
How many other vendors?
How many serving lines are you going to run?
How quick can you serve them?
How near an entrance are you going to be?
How many days is the event?
What are you serving, how long does it take to cook more of them?
The more information you can find out about the event the better you can make an estimate.
But I wouldn't want to anticipate more than 10% of any event crowd is going to eat at your stand, though they all may be dying for bbq and you're the only bbq vendor.
On multiple day events you can base day two off the day one sales, but you can be just as far out trying this method as any other.
If you are going to pre-cook anything, you can keep things on ice or under refrigeration and re-heat as you need it, otherwise cook what you can afford to take home, or shoot your load and pray you sell it all.
"There's no such thing as a little garlic" A. Baer
KCBS certified barbecue judge.