Appropriate Wheel Offset for 18" Wheels?

I also dont see how you think a 15mm offset will have any chance of fitting in the rear without getting SS/AA on your quarter panels. 25mm MAYBE, 15? No f-ing way. I ran a 10 wide with a 43mm offset and had about an inch of outer clearance WITH bulged and rolled lips. That was with a stock A-body rear with drum brakes, your rear is even wider???
The wheel I used for mock up was 17 x 8" and had a +30mm offset on it (measured and looked it up) with a bulged out 275/40/17 tire. I stacked washers up to about -5mm and had maybe .25"-.5" between the springs and .75"-1" between the outer trim depending on which side of the vehicle you were on. I don't see where my calculations are wrong. Rimsntires even states that the tire should be about .4" closer to the outer quarter panel which should give me adequate clearance. Am I right, or am I missing something here?