Hello mate.
It's a minefield isn't it?
You saw all the arguing on the MMA board about what fits what?
As I said on the MMA board I could not fit 245/15/60 tyres that were on 15x8 (4.5 backspace) on the front of my challenger without cutting the wheel arch lip and removing the brace inside the fender right above the tyre. less backspace (to make it miss the fender lip) would have ment it hit the frame rail at full lock.
However 245/14/60s DID fit no problem with the original 14x5.5 wheels.
It you are not 100% positive about 245s on a 15x7 wheel then stick to a 235. It is only 10mm narrower but I don't know of anyone with clearence issues with that.
Now to answer your question.
Backspace is seperate to rim size. The bigger the wheel the more backspace it tends to have as you can imagine but some wheels are available in different backspaces for the same wheel width.
Got my fingers crossed for you.