Ideal backspacing question

[RacingTowardsRed]

Hey all, quick question. I'm looking to get a set of track specific wheels, but the wheel I'm looking at doesn't give an offset number, but rather a "backspacing" number. My choices are 1", 2", 3", or 4". I've done some research but to be honest, offset and backspacing just confuses the crap out of me. Any opinions as to what would be the best choice for an E30? Thanks for any input!

Edit: sorry, probably not enough info. I'm looking at a 15" wheel, 8" wide.

[Madhatter]

Backspace is kind of like the other side of offset. It's the measurement from the wheel flange on the back of the wheel to the inner edge. It changes depending on wheel width. For some reason I'm thinking its a bit over 4" on a 7" wheel. I think I posted it once before

[Madhatter]

17x9, ET20, backspace is 140mm. Nearly 5.5". So you'll probably want to run more than 4" on a 8" wheel