Note the diagram on this page indicating where the measurements are taken to determine a wheels' offset. One thing I don't think is properly identified is what they mean by "true centerline" though, because I'm running what are considered 18"x8" rims on my car and when I measure the overall wheel width (height if wheel is laid face up on the ground) from front to back they measure 9". Knowing that, I think the 8" width measurements for my rims are actually taken at the outer edges of the tire beads.

At the same time, to measure the "true centerline" of a wheel, it would seem likely that the centerline would have to be based on the front and back edges of the wheel, although if the front and back beads are the same, I guess it wouldn't make any difference.

Anyway, now that I've probably confused the situation even more than it was, here's the page,

1010Tires Wheel Offset Tech

You can also use this calculator with the measurements you've taken to compare your rims.
1010Tires Wheel Offset Calculator