You space the disc out, so the caliper can then be spaced out from the fork leg mounting to give more clearance from the spokes to the back of the caliper, with disc then still sitting in the middle of the caliper.
To find the clearance you need to mount the caliper albeit if necessary remove the disc and space it out (say with washers) to see how much clearance you need to achieve to clear the spokes and it is entirely possible that your particular caliper cannot fit even if you machined material off the back which is another option.
The OEM one was a four piston fixed caliper, I doubt if a later Brembo one will give much real benefit!