I directly fit each case into a tabletop drill press, case mouth oriented up, of course. Power up & use a small needle file on the front of the rim. Light pressure is sufficient. I stop the machine & check thickness using a 45-60 shell holder. If it freely slides on, I'm done. If not, give it some more. I developed a good feel after a bit & was rather easy to hit it pretty close - I verified samples using a caliper. Has worked fine for me so far (a few hundred cases).
Regards,
Slim