rent is a case by case, depending on your area and what kind of location you want
selling by the slice is going to be location location location, since you need heavy traffic
ideally i kept my food costs between 27-30% on pizzas, with cheaper items like garlic knots down at 17-20%. i would adjust the price for my fixed costs
a problem with by the slice, is the same problem fast food places have, in that people expect it immediately, much faster than the amount of time to cook it. so you have to forecast how much pizza to make in advance, at what time of the day, so you dont have extra pizzas sitting there all day and you also dont run out.
i dont know about your area, but there was a lot more business in servicing local organizations like youth sports teams or local schools, either delivering to their events or coming to our restaurant afterwards, than there was in random walk ups. its also a lot easier since you know how much food to order and prepare in advance and you have guaranteed repeat business
a piece of advice is that most people order ingredients in quantities that are far too big, in that large inventory hides waste. its harder to notice one pizza worth of cheese missing if you have 500 cases of cheese etc.