I found this link for a offset calculater.
Its been the most helpfull to me out of all the links I found.

Alot of them give all kinds of details and measurments and etc. But I end up lost anyways.

This one lets you enter your stock wheel offset. And you can work off that to see how much strut clearance you lose and how much the wheel will extend. No formulas and etc...

Enjoy
http://marksink.com/tire_wheel_offset/offset.html