If you're running 30 degrees advance at 100o RPM it follows that you'll be running that much total advance. As a guide I'd say no more than 14 or so at idle, check total advance and you really shouldn't go beyond 39 to 40 degrees.


As ever it very much depends on how hard you run an engine as to whether you'll do damage. The fundamental problem with a mechanical distributor however is that the advance curve is essentially linear. In terms of using the engine safely what it runs like at idle is an irrelevance. You need to aim for safe settings under hard acceleration in order to gain reasonable engine life.


Regards



Pete