I provided that measurement in my post Jeremy. Approximately .200" of movement is all you need from the vacuum advance. This will give you roughly 10-12 degrees of timing at the crank. 20 is too much for your combo and will cause all kinds of idle issue on manifold vacuum, and you almost won't even be able to bring the idle down and cause throttle blade position issues as described above, which gets into all kinds of other tuning and drivability problems..
You won't get wonky idle problems and inconsistency if you set them up as I described above. Watch the first video and you'll see, that engine, even with it's 239 @ .050 cam, doesn't change in and out of gear more than 150 rpm.
The old canisters from the mid 60's on some of the solid lifter performance engines, like the B29 (if memory serves) were my favorite to use for these applications, because they worked with 6-7 inches of vacuum and were all done by 10-12 inches, and they didn't throw a ton of advance at you. For a while I was still getting these from Napa through the Echlin line but they have long been discontinued. Lucky for us the adjustable units can be modified to work in the same way.
Like I said lighten the spring tension as far as it will go. That makes the can start working around 6-7 inches. You then limit the travel as I have done in the pic below. I prefer to weld the slot. Then I file it until I have about .200" of movement. This provides 10-12 degrees at the crank. Simple.
If you have mechanical advance set up properly, you won't get any stumble, as the vacuum drops out with throttle input, the mechanical should be taking over in a linear fashion. At no point would there be "no timing" because vacuum dropped, it just doesn't happen that way. And there is no extra timing at full throttle because there is no vacuum pulling on the can. If a stumble is occurring something is fundamentally wrong with the setup.
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