Help with July 4th

Hi all.
Just bought our 1st cart and getting all the ducks in a row I.e Hd cert, registartion , llc , sales tax license. Now our firt time operating experience will be this saturday  the 4th. The community is over 80k and the festival is from 9am to 9pm and I'm at loss as to how much food i should by prior. My own logic tells me 80k people about 20k will show up and if 1% buy from me, I should sell approx 200 dogs,brats sausages etc... I plan to keep my wife on stand by to run to costco if i start running low. any input from others who have done a long 4th of july festival would be appraicaited. also I was trying to figure out pricing.
Thanks in advance

One thing to keep in mind is whether or not the venue allows people to bring their own food & drinks. I've found that events where people can bring a picknick wont buy nearly as much. If the venue wont allow people to bring food, then you'll sell more.

Oh yeah, and consider how many other food vendors are there to share the sales with. If its only a few (I would say 9 or less) you stand to be very busy. If its an event where they book way to many vendors (in my experience July 4th venues have a tendency to do this) then you'll sell less. I did a 4th of July event 2 years ago that was slammed but there was only about 8 of us there. The next year they more than doubled the vendors (had over 20) and none of us did any good. This year Im not even doing a 4th of July event. The 2 I did last year upped their booth fees & they wherent that good for me last year anyhow so instead, Im just doing a Horse Show that happens to be 4th of July weekend.

thanks, there expecting 15k people and theres only 3 vendors.
so we shall see.