To the OP, yeah, those faces should be more than 50% flat. 100% isn't necessary, I'd consider 70% to be a fair target.
Originally Posted by
dsaul
It is really a thing, but its really for facing post mounts on a frame, not an adapter that can be easily replaced. I'm not sure the cutter will be large enough to face the entire rectangular surface of the adapter.
https://www.parktool.com/product/dis...ing-set-dt-5-2
If it were mine, I would just clamp the adapter in my mill vise and mill it flat. I don't expect most people to have a machine shop in their garage.
The Park tool you've linked is likely the better tool for the job than your home mill. It's important that the post-mount flats are perfectly perpendicular to the rear axle, not to the welded disc tab.