So, I know this is kind of cheesy but I made a mock up wheel out of cardboard to see what offset I'd need for the rear. It's just a cut circle with a rectangular piece taped to the circumference to simulate the width of the tire. I measured it on both sides of the car just in case my perches weren't centered on the rear axle. The wheel is cut to about a 245/40/18 tire (may be a tad taller and wider). Overall, I got that I'd have at least if not more than a finger's worth of room on both sides (leaf spring to outer trim) with about a +7-8mm offset. I measured 130 mm from the outside of the inner "tire" cardboard to the inner face of the cardboard. 245mm/2 = 122.5mm -130mm = 7.5mm.