I ran into a similar issue this AM when installing an Avid BB7 Mtn S caliper on the fork of my recently received RadWagon. The OEM front disk had enough lateral runout to contact both pads during one wheel rotation, ~ .040" TIR. So I installed a 180mm Galfer Wave Floating Disc rotor, with orange buttons to match the Rad Power Bikes on my white Wagon's downtube. The peaks of the waves on this rotor were contacting the throat of the caliper. Aargh. So I have temporarily borrowed both pairs of spherical washers from one stock caliper SHCS and added one pair to each Avid mounting SHCS. This increased the standoff by 0.183", probably twice what is needed. So I won't be riding the Wagon this AM because I don't want to go through the bedding process until the caliper is properly located radially to just clear the rotor OD. The Galfer rotor runs true to less than .005", and it does allow slight lateral float. A short maiden ride yesterday has me pleased beyond my expectations with the ride & handling qualities of the RadWagon and somewhat allayed a pre-purchase concern with buying a bike that has only 1 appropriate tire that's available for it. So, the_natural, you may find that just increasing the caliper standoff slightly will allow you to use the Magura calipers.