Do you see that once the advance starts to move, the timing measured is no longer the initial.
Its now initial plus some advance.
The good part about the Mallory type advances is that you can pretty easily adjust them.
The uh challenge with the Mallory type advance mechanism is getting a curve that looks anything like the factory curves.
Before getting into adjusting, measure what it has now.
You have to know where it is now so you can at least guess at what changes should be made.
Get timing vs. rpm at the slowest rpm it will run, then every 200 or 250 or 500 rpm, measure timing again.
Leave the golf tee in the vac advance hose for all of this.
Only after you have the mechanical timing pretty ballpark is it ready for vacuum advance