Stoning the cam ring should be a very last resort. Really difficult to keep accurate as it depends on what part of the ramp the points contact - and a slight points gap difference uses a different part of the ramp. If points gaps are equal, it's probably the only remedy - or a new camring.
If they're unequal, the problem is either the insulating gasket the bearing is located in, or a badly worn / machined body that the camring sits in. The latter, however, is more likely with manual advance than ATD.
