I posted a thread a couple of weeks ago that has a link to a website which explains how to calculate required offset in relation to rim width and tyre size. Do a quick forum search using "offset"
I used this to choose my tyres/rims (285 75 R16 on 8" rim) and I only had to trim the tiniest amount from the front moulding so it proves it works (I have 3" lift).
Remember changes in offset can dramatically effect steering/handling, I ran my selection past a specialist before purchase.

Col