The $64 question. For the bottom of a magnetic chuck? When the top of the chuck will be ground afterwards?
More experienced grinder hands may very well have a different opinion.
Emo expo, machine tools, germany, where I was in 2012, with Haas.
They had a grinder that ground flat steel ..
to a perfect mirror finish, better than a bathroom mirror, and possibly close to an optical finish.
The samples were kept under glass, so we could not touch them.
I don´t know if You can do that, but grinding will definitely produce a mirror finish.
That can even be flat and straight, if your machine and technique allows.
Typical modern T&C cutter grinders, 5 axis, will likely make mirror finishes on anything, given the right tooling.
They cost 500k, so that might be a problem.
My clients used them to grind stamping dies, in carbide, to make tuna fish cans.
Interesting you are discussing a surface gauge. Someone gave me an import piece of crap surface gauge. Let's just say it was not stable on my surface plate.
I just finished making some lapping plates ala Tom Lipton.
I lapped this surface gauge to a flat condition. At least I can use it as it no longer moves and hinges properly.
A little elbow grease kept it out of the scrap pile.
Good to learn how to use a surface gauge for checking the squareness of a part, see youtube.
Here is an Ok Youtube, but does square off a ball. Perhaps see this guy's next youtube.
transform a surface gauge into a squareness comparator - YouTube