vacuum advance

Trying to figure out the ignition vacuum advance.
When I look at the shop manual for a -67 383 engine with 4 bbl carb, the vacuum advance should be set at 7.5 degrees at 12 " of vacuum. At what numbers do you guys set your vacuum timing advance or do you just tinker with it until best performance and gas mileage?
When I set the initial timing at approx. 22 degrees I get the total of 38 degrees with mechanical advance.
My reading of the vacuum at idle is 12" with the MP purple 284/484 cam.
With other words, how many degrees should the vacuum advance be set to.

Another perhaps stupid question, when reving to 2500 rpm and holding the engine at that rpm, is the vacuum advance activated at that rpm?
Guess I can connect a vacuum gauge to see how much vacuum I´ve got at 2500 rpm, right?
/Thanks, Lenny