Two things need to be done to properly align the new rear lower quarter patch panels. You need to install the rear valance and the wheel opening moldings. I'm serious. There is no measurement, especially one involving the tires since there are thousands of variations. The first time that I installed a set years ago, I installed the rear valance panel, but not the wheel opening moldings. You would not believe how critical this is for the w/o moldings to fit. If you don't have the moldings now and decide to put them on later, you might be screwed as far as proper fit goes. Real world experience talking here. The patch panel may be in slightly in the front and look absolutely perfect until w/o molding time.
Just tack the panels in place until you're sure that the fit is right and then weld em up.
