The rule of thumb is normally to START at 34 degrees (initial and Mechanical) all in by 3000 rpm. If you can you go more before detonation, great. 34 degrees is a good starting point. Do all of your testing with the vacuum advance disconnected.
Once that is done, then if you have an adjustable vacuum advance, START with adjusting to 48 degrees of total advance (initial, mechanical, and vacuum) and try it. If any detonation, back down, if none, then move up, but to no more than 52 degrees. You just have to test and find out what is best for your combo.