Problem installing disc caliper

If someone can tell me what I'm missing, I'd appreciate it.

Here's the deal:
I'm attaching a Hope Mono M4 disc caliper to a RockShox Reba Team fork (ISO
disc). The front hub is Chris King ISO disc. The caliper comes with
several washers to shim it toward the center of the wheel, to get it
centered over the disc, but my problem is that without any washers, the
caliper is already slightly too far (about 1-1.5mm)toward the center of the
wheel. This is causing the rotor to rub on the caliper body a bit (noise,
drag). I can't see any way to move the caliper away from the center of the
wheel without having some sort of machining done on either the ISO mounts on
the fork, or on the mounting areas of the caliper. Is that the only
solution?

TIA,

Cal