What is the correct amount of vacumn advance?

Here's an easy way to determine the corrrect initial advance for your particular setup:

Start the car and let it warm to operating temperature then shut it off.

Advance the timing 2° and let it sit for 3 -5 minutes to allow the temperature to spike.

Fire it back up. Did it pull the starter down like the battery was weak? If not, repeat the process until it does. Back the timing off 2° and lock it down. This is the maximum advance that your setup will handle and that is what you want.