Warren, you're not concerned about the edge of the wheel...you're concerned about the center of the wheel, which determines where the edge of the tire is. If the wheel center moves, the tire moves.
This can get a little confusing, as you can see. That's why I designed the calculator to calculate the track based on wheel mounting width and backspacing or offset, and the clearances depend on tire size, not wheel size. Actually I use offset for the calculations, even if the user enters backspacing.