How much backspacing do I need to correctly fit 33"x12.5"s on a yj?

also, just to clarify, I know that backspacing is like the offset, but what exactly does it measure? what is further out from the vehicle, a higher, or lower number?

I have never had to worry about this before, because last time i lifted a jeep, i already had rims and tires, so pardon me if it seems i am asking stupid questions. <img src="/forums/images/graemlins/kewl.gif" alt="" />

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94 YJ. no lift...YET