I have 180mm rotors available (Hope and Avid)
New forks with Post mounts.
XTR M965 post mount calipers.

My LBS has a Hope adaptor model “H”, bottom right of this lot:

It is specifically for a 183mm rotor.

Assuming that mounting the caliper at 183mm on 180mm rotor is not ideal due to the braking surface not covering the whole pad face, if I were an impatient bugger and didnt want to wait for a 180mm adapator, would it be perfectly reasonable to file 3mm, say, off the adaptor legs instead to mount the caliper just that littl ebit closer to the axle centre?

I did this to a avid 185 caliper to bring it down to 183 but you only want to take 1.5mm off as the size is the diameter.

it’s not as simple as taking 1.5mm off as the mounts are not radial to the disc. you need to some geometery to work out how much to take off.

doh, 1.5mm, yes I see!

If its so little Ill just do it bit by bit until it fits I reckon.