So, if I want to match the placement of the current wheel (15x10 5" bs with 1/2" offset) with the narrower 18x9.5, then the 5.25 backspacing with 0 offset would be correct - right?
No. That would move the inside edge of the rear wheel 1/2" deeper than where you are now. If you are rubbing on the inside now that would make it 1/2" worse. Not what you want.
If you want the 18x9.5 centered in the same space as the current wheel you need to maintain the same -1/2" offset. That would be 4.75" backspacing. If you are rubbing on the tub and the fender lip both this is what you want to do. Worst case, you decide you want to space it out a little (less offset) and you can run a thin spacer if needed.
