There are two active factors to how the wheel will sit off the hub, back spacing and wheel width which between the two dictates the actual off set mathematically
Back spacing is the amount of space from the back of the rim to the mounting point
Offset Is how far in or out the mounting point is from the actual centerline of the wheel and is measured in millimeters (mm) a minus offset will push the wheel out, a positive offset will pull the wheel in a bit
A typical back spacing for our trucks will be somewhere between 4.5" and 5 3/4" on up to a 10" wide wheel, over 10" I'm not so sure...but I'd guess that for every inch over 10" wide you'd go, you would need to compensate a minimum of a 1/2" in back spacing, so a 12" wide wheel would need to have something between a 5.5" and 6 3/4" back spacing to fit one of our trucks, and on a 12" wide wheel that would be something like a -25mm to a -55 off set, depending on back spacing of course, and I'm pretty sure the truck would need to be lifted to fit a wheel that wide
The offset is set by the width of the wheel and the back spacing of the wheel, it's all relative, if the back space moves so does the off set, so long as the wheel width stays the same.
If the link works, here is a pretty simple video explaining it
http://m.youtube.com/watch?v=2K1lUlv9ZOA&app=m