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The vacuum advance is ported vacuum on these old sixes the best I remember. Which means for there to be any vacuum to the distributor, the throttle plate should be off idle and least enough to allow vacuum to pull on the the advance.

You are right, the vacuum will be low, depending on how the engine is revved.

The best way to feel it is to actually goose the engine a bit.

You are also right that at idle the vacuum would be highest, but with the throttle plate closed the port to the vacuum advance line does not get any vacuum.

At higher loads and speeds the vacuum advance actually does very little on these sixes so long as the throttle is wide open. The mechanical advance does most of the work as the engine speed increases.

Pulling a long steep hill under a load with the throttle wide open would leave no vacuum to work the vacuum advance, so the mechanical advance keeps the timing up.

At least that's the way I was taught it worked.

If I'm wrong, ignore all I said. LOL