Manifold Vacuum Advance what am I not getting?

Thousands, if not millions of GM cars, came with MVA. Below is a page out of the 1967 Pontiac manual stating the use of MVA. These cars had 6* initial & 20* MVA added so that they idled with 26*. Small cam, 197 @ 050, 10.75:1 CR.
What would the mechanical curve on this set up look like?