Wheels & Tires - ls1 15x10" back spacing??

hey, im looking to put a set of prostars on my formula and was planning on running 15x10's in the rear and 15x3.5's in the front. i know about all the wheel spacers and the studs and everything else.. i was just wondering what back spacing would i have to get inorder for the 15x10's to fit? what is the stock wheel back spacing? thanks alot guys.

7.5 for a 15x10 there is so much info on this in the stickys and in the search.