I remember asking this question earlier on. There is no real formula, just experience. I do remember thinking how many brats I would sell each minute, then hour, then # of hours working the event. Of course come slow and fast times. I totally guesstimated on dogs, brats, and spicy polish. I came really close to running out, but not by much-perfect. If it’s a gig that goes until late, keep open if you can, a lot of vendors close early. We stayed open and made a killing on the beer drinking music dancing drunkard-style crowd. Remember lights. I am getting the festival fever just thinkin about it.