Ther isn't a "best" backspacing, it all depends on what you want. I would reccoment 4.5" or there abouts. That will get the tires away from the body but not so much it looks wierd or hs the potential of causing handling problems. The farther the wheel centerline is outboard, death wobble might happen if you have some slightly worn suspension or steering components whereas a wheel with a centerline closer in-board could not have deathwobble. In my opinion, any wheel with a backspacing numerically lower than 4" looks bad on XJ's.