Another annoying Backspace question

Ok, did the searches, found tons of repeats, but I still don't understand the whole backspacing issue. I am in the processess of installing a Full Traction 6" lift on a 99 xj and am planning on running 35x12.50's and I'm still not sure what the correct backspacing should be. I hadn't found any that mentioned tires rubbing on the LCA's on long armed kits like the FT has, and my question is, what backspacing will give me decent stuff (I'm going to bumpstop and try to reduce the ammount of trimming I have to do) but will still prevent my tires from rubbing on the LCA's? This is my DD and I am in Northern VA if that helps you out any as far as tires sticking out of the wheel well's go. Thanks a bunch guys.