[martrogers]

This does not seem to be your problem but worth mentioning anyway.

When working out my offsets/backspace I realised from the rim cross section drawings I was sent that whilst offset is based on the rim width which is measured between the inside of the rim flanges, backspace is measured to the outside of the inboard rim flange. In my case the rim flange (and its flare) was 13mm so this needs to be taken into account when converting offsets to backspaces if you're tight for space.