What do you mean Otto? Backspacing and offset are the same thing, just presented relative to the centerline of the rim vs the back edge of the rim.
You have to consider both the offset/backspacing and the width of the wheel to determine where the tire will sit relative to the frame/fender.
The advantage of using the tool Curt mentioned to identify where the tire will be vs the wheelsmith calculator is that you can turn the steering with the tool and check clearances at full lock where they are the lowest.
