I measure out to .20, scribed a line. When it was installed in the brake, I more or less eye balled how thick the front edge of the brake was, I then set it back approximatly that far back to make up for the radius. My scribbed line was as close to the center of the radius as I guess it can be.
I am no machinest by any means, just fly by the seat of my pants most times...
Hope that helps.