What is the correct amount of vacumn advance?

Perhaps there a few different issues being confused in this thread - I think they are:
initial (starting) timing
total mechanical advance
vacuum advance.

Doing this justice would require pages but let me try the "highlights".

Most folks in this forum would probably be most concerned with total advance as that is what your engine sees when under medium to high rpm load - it is what directly impacts HP and torque. Others in this thread mentioned 32 to 36 and that is general correct but there can be fairly significant power differences between these numbers. It is set by disconnecting the vacuum line and reving the motor to engage all of the mechanical advance that you have dialed into your distributor (usually ~3000 RPM gets it done). Another way would be by subtracting the mechanical advance (usually ~18 degrees) from your desired total (say 32) to get the the idle or start advance (32-18= 14 degrees at idle). I prefer the first approach but many use the second.

Now the tough part ... what should the total (32 in this example) be? Best answer is probably a dyno. Issue though is as soon as something changes (like jetting) - you may get a different answer. My current stroker likes 33 and that did not seem to change much as we changed jetting but other motors I have dynoed really required less advance as the motor leaned out.

Second way is at the track.... probably a lot more fun!

The primary issue with initial advance is making certain it is low enough so the motor starts. In this example of 32 total, 18 in the distributor and 14 starting: if the car was still hard to start when hot you could add more advance inside the distributor (there are stops or bushings stops that can be changed in many distributors) to lower the initial or idle advances. Fir example you may go to 20 and therefore only 12 to start the car (32-20).

Vacuum advance comes t play under part throttle - call it economy load. Since more timing can be applied under light loads (when there is higher vacuum) and it helps economy - the distributor advances the timing under those conditions.

Kory