Ok, I found this a while ago while I was wheel shopping
http://www.morfed.net/wheel-offset-calculator/
You can put in the offset of your current wheels, then put in the offset of the new wheels (including witdth difference), it'll then tell you how much more proud the rim will be from the arch.
The width varies the offset, because the offset is a measurement from the hub face of the wheel, the wider the wheel, the higher the offset for the same out edge measurement.
Hard to explain in words, (probably even harder to understand), but play around with that link and you'll soom get it.
For an 8.5" rim with 35 offset, moving up to a 9.5" wheel, to keep the same outer edge, you need a 48 offset.