I think the string thing is inaccurate enough (try it!) that you might as well just eye-ball it. If accuracy mattered, I would make a paper circle of about the size of the pizza, divide into twelve (sixteen would work too) slices, remove one (or five), and adjust the spaces between them until they looked about equal. Then slice between the paper pieces. If I were in a pizza shop, I'd probably just use actual pizza slices.