Originally Posted by John_01
The calculation you wrote isn't what really determines if the tire will rub. The offset is much more important than the wheel width - unless you are using a really stretched tire. Generally its the shoulder of the tire that is most likely to rub depending on the tire width and wheel offset. The 235/40 on the rear should be fine though. Hopefully you won't have any more issues.
My calculation is half the wheel width minus the offset, so I'm taking both into account to determine how close it would come to the fender edge.
I'm not having any issues with what I'm running now. Next time I'm due for rear tires, I'm going to try 245/35 and see if that works.