Vacuum advance information and myths

Initial timing (seen on the crank at idle), Mechanical Advance (measured at about 3000 RPM or when springs are fully extended), and Vacuum Advance all contribute to total advance which must vary based on engine speed and load. Best power on a SBM is usually achieved at 34° Crankshift advance. Detonation is the limiting factor, but maximum power (MPH in the quarter is a good measure) is the goal.
Using best power timing, know the mechanical advance amount in distributor degrees (usually found on the distributor advance plate shown in Distributor Degrees). Initial timing will then be Total Advance minus 2X Distributor Advance. I have seen electronic distributors with 11, 12, 14 and 15 degree advance plates which means 22 to 30 degree crankshaft advance. Taking the 15° advance plate example, total timing of 34 degrees less 30 degrees means initial advance of 4 degrees. This is fairly typical for many pre-pollution controlled cars.
To get more initial advance which often helps idle quality and engine idle temperature, you need to reduce the mechanical advance to say 11 distributor degrees. This will allow 12 degrees initial advance and 11X2 or 22 degrees distributor advance. The FBO advance limitor plate is a good way to handle this issue. FBO Mopar Distributor Limiter Plate
Total advance at cruise conditions is controlled by the vacuum advance canister. The degrees advance are on the arm inside the distributor so you need to remove the canister to see the value in distributor degrees. Double that value will be the crankshaft degrees the canister will deliver. The desired advance is that value which delivers best fuel economy without detonation on tip-in acceleration (usually the worst condition for detonation).
I set up my 340 this way back in the '70's during the Fuel Crisis and got 24 MPG out of the '68 Cuda with careful driving but still durned 14.5's with 2.94 gears.
Good luck with tuning your ride!