This is a problem with the swinging caliper mounting design, which doesn't allow the pads to sit flat against the rotor until they wear in. Remove the adjustment screw and spring, and loosen the three bolts that mount the caliper to the fork leg. Back them out a fair amount, to allow the caliper to float. Squeeze the brake lever, which will force the pads flat against the rotor, and note the gaps between the caliper mounting bracket and the fork. This is the thickness of the shim washers that need to be added. All three shims should be the same thickness.