Thought I'd let everyone know about this...

http://toy4two.home.mindspring.com/offset.html

Provided you know your stock rim specs and offsets... and the specs of the rim you're trying to put on there.... it'll give you the exact measurements of how much farther it will stick out as well as how much room you'll have behind the rim (suspension clearance)

You can use this to get the PERFECT offset depending on the look you're going for.