Manifold Vacuum Advance what am I not getting?

The way I read this.......

22* Base timing
12* Mechanical timing (restricted to)
20* Manifold Vacuum

Cranks at 22* and allows to idle at his preferred 42*
Step on it and the timing drops back as the vac is lost hopefully no ping (or that's the idea)
As rpm goes up, timing advances to 34*
Lay off the accelerator and the 20* MV comes back to cruise.

Of course, curve rate would have to be tailored for the application.

Just my 2 cents.