I'm not really understanding that Byrne. Manifold vacuum is at its greatest when the car is at idle (but we block it from advancing the dizzy). The mechanics of the unit inside the distributor control how much it can advance the plate, and our book says 20* is the max, when the vacuum is greater than 15" of Hg "pull". As the throttle is opened, manifold vacuum drops as a result of being exposed to atmosphere (inches of "pull" goes down), so I guess I'm confused as to why you say the vacuum gauge would show more vacuum?
Maybe the difference is in manifold vacuum and throttle body port vacuum? With air rushing faster past the holes in the throttle body it would increase the venturi effect and lower the pressure for sure. The manifold vacuum isn't subject to that same effect because of the larger volume and slower movement of air.
In a stock Delorean the vacuum hose to the distributor comes from the thermal vacuum control valve and the vacuum source for that valve is the manifold itself, not the throttle body, right?
Or perhaps you were suggesting setting the static timing via vacuum gauge reading rather than timing light? Sorry, I'm confused as to your point.