There is a lot of misconception about how all this works, because of you either view it from a performance perspective or from an emissions perspective.

For performance, the vacuum advance MUST be hooked to a port below the throttle blades for it to work correctly. If the port you are hooking it to is at or near "0" until you rev the motor; you have it hooked above the throttle blades and its on the wrong port.

You want it hooked to a port that has full vacuum at idle and subsequently cruising speed. When the throttle is opened, the manifold vacuum will instantly drop towards or all the way to zero depending on how far you stomp on it.

At that split second, all the added timing from your vacuum advance that was maxed out at idle and/or at cruising speed drops to no added timing via the vacuum advance pod. It saves the motor from detonation as you roll-it-on harder than a mild acceleration, floor it, go up a hill at heavy throttle under load in too low a gear, or just punch it.

At that point, only your mechanical advance, along with your initial timing is dictating your total advance.

It's easy when you think of it this way.

Also, what ever your cruise rpm is on the highway, ideally (if you don't have too much compression) all your mechanical advance should be all in (READ: use the correct advance springs to allow full advance by your normal cruise RPM) When you are at light cruising speed of the freeway or just idling in the driveway all your vacuum advance will be in for maximum fuel mileage and dramatically increased off-throttle responsiveness. Typically, your timing will be up around 50 to 52 degrees total with the vacuum advance hooked up and around 32 to 36 total when it is not. There are a lot of variables at play that dictate those final optimal timing choices like fuel quality, compression (both static and dynamic, altitude, combustion chamber and piston design etc, etc.