Thanks for that.
Let's see if I can explain why I set it the way I do. I know I won't convince you! But maybe it will help someone else understand what I am doing.
When the engine is at idle, the car stopped, and you tromp the throttle to launch the vehicle, there is almost no air flow through the engine, so no resistance through the carb and air cleaner and all, and the manifold pressure goes to 1 bar. At the same time, the engine has high load, because the drivetrain is at dead stop. The cylinder pressures go way up. This is the most likely point where an engine will detonate if the timing is too advanced.
For racing applications, they avoid this situation by running a torque converter with a higher lock rpm. The engine gets to wind up before it sees the full load of the drivetrain. This also lets them run cams that don't make as much torque down low; they wind the engine up to where the power band is before the torque converter locks. High-rpm torque converters are also a PITA to drive on the street, and put high streeses on transmissions.
In a street application, we have the engine at low rpm, the manifold at one bar, and the drivetrain load on the engine is a maximum. This is where detonation will occur if ever. The maximum power you can get from the charge in the engine at this point is with the earliest timing that will not cause knocking. This is why all the old geezer hot rodders and engine tuners I know say to advance the base timing until you get knocking, then back off 2 degrees to give some safety margin against variations in gasoline, atmospheric conditions (temperature, humidity, barometric pressure), and other variables.
As the engine speeds up, more air flows, and the induction system introduces some pressure loss, the vacuum comes up a bit and the cylinder pressures come down. The timing, which is set in degrees, gets shorter in milliseconds: 15 degrees of advance is 2.5 milliseconds at 1000 rpm, but is only 1.67 milliseconds at 1500 rpm and 1.25 milliseconds at 2000 rpm. If we already determined that 2.5 milliseconds was the detonation limit at 1000 rpm, then we have some head room now. We can add advance -- 2.5 milliseconds at 1500 rpm is 22.5 degrees, 7.5 degrees more than the base timing of 15 degrees, and 2.5 milliseconds at 2000 rpm is 30 degrees, 15 degrees over the 15 degrees base timing. The mechanical advance adds the extra advance we need to keep the engine in time but stay short of the detonation limit.
That's with the throttle plate wide open. Closing the throttle plate adds air resistance in the induction system, resulting in a lot of manifold vacuum and reduced cylinder pressures. These thinner mixtures raise the detonation limit a lot, because thinner mixtures burn more slowly and because compression heating of the charge is much less, and so we can add a lot more advance without stepping over the limit. The vacuum advance monitors the intake manifold and if set up properly adds additional timing based on the lower cylinder pressure.
This doesn't affect performance -- torque from launch through redline at wide open throttle conditions -- and so race cars don't run vacuum advance. Extra parts to break, and if it sticks advanced, the heads will go through the hood on launch. For a street car, though, where you are usually not at wide-open throttle, the vacuum advance affects driveability and mileage. Setting up the vacuum advance for smoothest idle, and setting the rpm it comes in at to enable vacuum advance at constant-speed cruise, should result in the best driveability and mileage. Having the vacuum advance drop out too early (higher number) will reduce mileage at cruise, having it drop out too late (lower number) risks going over the detonation limit in transition, such as cruising at speed and then stomping the throttle to pass.
Anyway, that's why I set mine the way I do.
