Vacuum advance can adjustment

I think I know what you might find confusing:
Inside that can is a spring and a preload adjuster. When the spring has no additional preload from the tensioner, then vacuum has an easy time to pull the timing in. When the spring receives maximum preload, vacuum has to go to a higher level to pull it all in.
For the most part, that little screw you turn, only adjusts the preload, and
When you turn the screw CCW you are reducing the preload, and that always confused me,lol. The screw is peened on the end, so it can't come out unless you really force it

The amount of advance can only max at what is on the arm, but only if your engine can pull the vacuum to do it.
At one time there were a myriad of cans available to suit nearly any engine, but they are getting harder and harder to find.

The step on the arm limits the advance. You can cut some step off if you need more advance. If you have too much, you can limit the plate travel inside the D. But you cannot mess with the spring calibration, outside of the provided range.

Generally, the vacuum required to max the advance, is somewhere between 9 and 20 inches, and almost any street engine can achieve this by cruising rpm, and that is when this device really shines.

Really the preload adjuster is just to delay the timing change to prevent detonation at low-rpm/part throttle, on engines that are susceptible to that condition. I bring mine in as fast and hard as it can.