formula99-ls1-ws6

Im sure this question has been beat to death but what backspace do i need if i want to run 4" runners on the front of my 99 formula i know that i need a 5.5" for the back with a 15x8 wheel but not sure on the fronts..... also i plan on getting the summit drag thrust wheels there cheap and look good if anyone is running these wheels how do u like them??and how big of a tire can i run thanks in advance