My bike, as stock, came with 200mm sram rotors for guide brakes. I have some 203mm shimano icetec rotors, and thought about a swap.
The sweep will, therefore, be 1.5mm “out” on radius so does this need compensating for with washers, and is it good enough to get it near enough, “by eye”, checking where the pad hits the rotor braking surface?
Doesn’t sound a lot said out loud but safety first and what not…
Edit, the calliper is already on an adaptor, so easiest to swap that too a 203mm shimano rotor is is the bite area of a shimano pad, in the calliper, set different to those in a guide?
Stick it on and see how the tolerances add up, if the rotor spins then it's fine. If it fouls the caliper and doesn't then add some washers untill it does. Doesn't need to be any more exact than that.
I've run exactly that combination (guide + ice tech rotors) on my scandal, and similar mismatches on my other bikes (e.g. hope 205 with Shimano, or bodging missing avid CPS washers (which make them 205mm) with 203 Shimano rotors and washers.
It might make a difference on a heavily worn rotor with a lip, but on a new-ish rotor it won't matter if the Shimano or SRAM pads were different sizes.