To Vacuum Advance or Not to Vacuum Advance

In a hi-performance street engine;
the sparkport is far enough up the side of the throttle bore that in neutral, with the transfer slot synced, it will take something like 1500-1700 rpm to to put vacuum to it... in neutral, with no load; and of course the secondaries are closed. Of course as soon as you put it into gear, this changes.
If you continue reving it up still in neutral, the vacuum will plateau at some number that is highly cam-specific, somewhere between say 1800 and 2200, and I'll guess in the range of 20-22 inches. After this plateau, the vacuum will begin a slow downwards march with rpm/ throttle opening. Load changes everything. Idle-timing changes the vacuum onset.
The tuners job is to match the Vcan's operation to the application, in; the amount of the timing assist, the onset point, and the rate of advance.

Say you had a 360 cuber with a 220*@.050 cam. and the compression maxed for your combo. This will make a stinking torque monster.
Say it's an automatic with a 2200 TC . And say your T-port sync landed you at 14* of idle-timing, with a two stage timing curve that starts advancing at 800 and stage one stops at 28*@2800rpm. So the mechanical advance is 28 less 14= 14*, and it takes 2800 less 800=2000rpm for it to all come in. Ok, that maths out to 14/ (2000 divided by 100)= .7degree per 100 rpm. Now you can calculate the advance anywhere between idle and 2800rpm.
Ok So, at 2200 you will have 24* mechanical timing. Since your brakes will not hold the car at WOT, say you opened the throttle to just before break-away and the vacuum jumps to 22inches, and 22 appears at the spark-port..... and your Vcan is set to bring in 22 degrees more timing at 22" of vacuum, so you add those 22, to the 24 mechanical... for a total of 46*, see how that works? Badaboom, you have a torque monster. Now; as soon as the tires break traction, you floor it and the vacuum decays , and the advance drops out, and so as the engine climbs up the torque curve, it is dropping timing to keep itself out of detonation.
But say the 46* gets you into detonation with your iron heads. Well you just get a different Vcan that brings in only 9*say, and your total is now 24+9=33*, and so on it goes. If you think you can use more, you mod the Vcan, by cutting the stops, to get it....Most Vcans I have seen,can be modded up to 22 even 24 degrees.
To match that with mechanical only, you would have to set the mechanical plus base(idle) timing to 33* at 2200, and see how your iron-headed, 220-cammed 360, likes all the timing it takes below 2200 to achieve the 33* at 2200.

To figure out exactly how much timing your combo wants at stall, first you need to figure out what signal you are getting at the sparkport, and then you need a dash-mounted, dial-back, timing delay box, to start playing with it. Sooner or later, if you start logging data, you will get a picture of what she likes. Then you go make it happen,as best as you can.
Or you can just keep throwing stuff at her to see what sticks, but I can tell you that pulling out the D three times in 5 miles is a PITA.

The above is a theoretical situation. Yeah I ran that setup once but with a clutch and aluminum heads,lol and 185psi cylinder pressure.

Now; with 3.55s and 27" tires; 65=2870 rpm, and the timing is 28 mechanical (with 14idle), and the Vcan is maxed out at 22* so the engine is seeing 50* for cruising; this may be too much, but probably still not enough,. But if you add 22* to 36*, (if you had curved your D to all in by 2800), then 58* might be too much;
only the dial-back timing device will sort it out for you.
Or endless trial and error.
Happy HotRodding