Bruce, that number is kind of a baseline to start from. It should allow the car to idle reasonably well, and depending on the combo the engine might tolerate or want more. That can be from more initial if the total advance isn't too much, or from the vacuum advance as you suggest. No real hard numbers, just what the engine likes.

With a big cam, the engine will do less "hunting" if it's running just on the initial timing (ported vacuum). With manifold vacuum, if the engine speeds up slightly, the incease in vacuum may also increase the timing enough to speed the engine up, which can increase the vacuum advance, which speeds up the engine, which can start the mechanical advance to further complicate things.

Usually the problem is that the idle speed is set in neutral and both mech and vacuum advance are somewhere in the middle of their range. Then the car gets put in gear, the engine slows down, and the timing backs down, and everything drops off until the only way to keep the car running is the "two-step".