I have only worked festivals and special events in the past. Nothing for the last three years. I am in the process of returning to the game. (so take this with a grain of salt)
However, what I have found in the past is that
promoters are very optimistic. Lets say for example they expect 30,000 attendees per day. I always half that now we are down to 15,000. Then I say half of them will not purchase food, so now we are at 7500 attendees. Now I divide that number by the number of food vendors lets say they have 20 food vendors (remember they are expecting 30,000). That gives each food vendor a potential of 375 folks buying something from them that day, fries, drink, dogs, burgers, pickles, whatever.
And this is only if all the stars in the sky line up and everyone is sing "we are the world" and it doesn't rain.
I know not very statistical like, but if I hit it right once out of three times I'm happy with them odds.
Any formula is better than "Not a Clue" heheh hehe heheh Hope this helps