You want minimum .020 over bullet diameter.
My experience is that overbore isn't as critical to performance as some think.
Plenty of suppressors are used universally to fit .22 cal and .30 cal. without significant loss of performance.
If muzzle work (crowning) is done through the headstock, I fit the brake to the muzzle after threading, make a light cut with a boring bar to assure the drill bit (and after, reamers) is following a concentric hole. All done in the same setup.
If the muzzle work is done between centers, it may be "technically" not as precise as you can't assure perfect concentricity in a single setup. I drill/ream the brake to size in a collet. Any from a reputable manufacturer will have the threads and center hole concentric to the OD of the brake, and I've never had a problem doing it this way, either.
Never had the bug to want to machine my own from scratch, can be bought much cheaper than I can make one...
If going down that road, this would be a good read, and they make "blanks" to save some work if you just want to experiment with DIY porting:
The Accurate Muzzle Brake Book – Newlon Precision