Your statement is wrong, or at least the way I understand what you said
All advance is ADDITIVE
You start with initial, "whut you set" with a timing light at idle.
You ADD mechanical which is purely a function of RPM, the springs, (for rate) and the length of the advance slots (for amount, or degrees).
So at WOT foot on the floor, RPM high enough that mechanical is "all in" you have "whatever is in the mechanical" added to the initial.
LAST
vacuum is ADDED to the top two AT WHATEVER POINT the mechanical "is."
This means that if you are at a fairly low RPM (stock engine) and the mechanical is not "all in" but that if throttle conditions are "right," you might have ALL the vacuum can ADDED to ONLY PART of mechanical.
On the other hand if you are at a higher RPM, and the mechanical is "all in" and IF throttle conditions are "right" you then might have ALL the mechanical added to ALL the vacuum.
As you give it more and more throttle, the vacuum drops out more and more.
If you start looking at specs "in the book" keep in mind that these are DISTRIBUTOR degrees which are 1/2 "crank"
Likewise, some mechanical "top hat" assemblies are stamped with a number, like "15" This is THIRTY crank degrees.