well, there is no fixed figure or formula for the industry. its up to each corp. to decide what they require. most small mom and pops go with hoeps and prayers. and good food, of course.
i would think going in, there has to be some calculations involved. before we opened our place i calculated what we need for income and worked backwards to give me a 'survival customer count'. in my case, i figured my daily customer count would be low(due to location), so i opted to sell bigger portions. the idea being that i could turn one customer into two, by selling half pounders instead of quater pounders....
a corporate place like sub shops, would most likely expect 200 guest a day during the week, and 3-4 hundred on the weekends, giving them average weekly sells of 10-15 thousand.