How do we adapt your method to Arbor boards with Griptech? Do we draw an imaginary line alone the non-Griptech sidecut and compare foot measurements to that? I don't think we are supposed to have barefoot overhang over the Griptech bumps.
Grip Tech will rarely require any different adjustment. The symmetrical humps that Arbor uses are very subtle and are smaller in dimension than typical binding adjustments will allow. Magnetraction uses asymmetrical shapes many of which also fall beneath the refinement of binding adjustments. Some of the newer Magnetraction humps we are seeing going into 2013 such as on the Niche snowboards are actually pretty deep and the humps are also quite short. Your foot may not fall "all on" or "all off" of a hump on either toe or heelside (or both) depending on your stance angle and width. In those instances you will still need to have at least partial barefoot overhang at your position. As always, centering the boot (not the binding) on the centerline of the board is the ideal.
We stress this a lot, but toe drag is only one issue in terms of width choice (and is not the most important). Being able to achieve a centerred stance with adequate and balanced leverage is far more important.