Hope this helps out explain the adjustable vac advance. It does not adjust the amount of advance it adjust the rate of advance.

This should help you can see when the screw is all the way out CCW, it compresses the spring in the vac advance, usually takes over 20 INHG to move the diaphragm when it is out all the way CW they usually start moving at around 5 INHG, hope it helps explain it.