the 5% rule seems to work. there are several discussions on festivals in the search section.
but a couple of things to remember, every festival I have attended , and they have been hundreds, inflates the attendance numbers, so be careful how much you pay to enter a festival as a food vendor. you may be able to get fairly accurate numbers from the local police dept. Every promoter has to provide security for the festival, and that is based on 1 officer per x number of attendees, there for the police department should have some fairly accurate numbers, the promoter is less likely to inflate the numbers to them because it cost them money when they do, also the Port a potty co is a good contact as the same rule applies to them.
One flag that came up in your post is “few other vendors”
why is that, find out. How far off is the show, another flag is a show that big that you can get into on short notice.
It is hard for a cart to compete with a trailer, it can but it is hard. Find out how many of the other vendors will be allowed to sell hot dogs, just because they listed funnel cakes on their app, doesn’t mean the promoter won’t let them get away with selling hot dogs, hot dogs.
also you may be in competition with a local church, or cheerleader group selling $1 hot dogs.
I’m not trying to discourage you here, we had a great and free life vending for over 30 years, wish I could do it for 30 more. show vending is not about “the show” it is about which show, do the research and home work on each potential show and you will do great. good luck mcw