Hooking up vacuum advance to manifold vacuum is adding in advance, but it's doing it at a point in load/RPM where you should be altering springs on the advance weights, or dialing in the distributor twist.
Not saying this is the only way or even the right way but this is how I would approach it.
First I would find the timing curve that the motor wants for full load wide open throttle. lets say at 2500 it wants 25 degrees at 3000 it wants 30 degrees and 3500 to 5000 it wants 34 degrees.
Next would be all the part throttle and cruise stuff.
Last would be idle. Lets say for example the motor wants 40 degrees at idle. If I am using mechanical and or static advance to set the idle to 40 degrees then the rest of my curve for full load wide open will be way too high. So to get the 40 degrees that the motor wants at idle I can use manifold vacuum on the vacuum can. I will have to adjust the vacuum advance for rate and the maximum stop accordingly. I also will have to review my timing table at part throttle to make sure the timing added by the vacuum can does not exceed what the motor will tolerate any where in the table. If there is a place in the table where this occurs I may have to compromise my idle timing and or elsewhere to a lower number. This is the limitation of mechanical/ vacuum advance type distributor.
There are distributors/ spark boxes/ fuel injection computers that alow you to do this digitally allowing you to set up curves with out these limitations.
Just for the record different combinations require different curves. Not all motors are happy with manifold vacuum. Some will want ported vacuum ( another name for manifold vacuum that only occurs after the throttle plates have been cracked open a bit). Some engines will be fine with no vacuum advance at all for example old tractors and some race engines. Some are fine with no mechanical advance for example small lawn mower engines and again race engines (sometimes fewer parts that can fail and simplicity takes precedent).
It all comes down to application and what is acceptable for engine efficiency.
Yikes, way too long!
