[Trackrash]

I think you are over thinking it. But here are my thoughts/experience.

First the original purpose of a vacuum advance was to aide in better idle and light cruise conditions. I other words when the throttle is closed or only slightly open there is a higher vacuum in the manifold. Most motors, at least before emissions and electronics, benefited from more advance when at idle. Also during a light throttle cruise more advance helped due to less fuel being burned and perhaps a leaner condition could be tolerated. Once more throttle was added and more load on the motor less advance was wanted or needed so the vacuum advance was reduced and the mechanical advance in the distributor took over.

However the older 911 motors did not have manifolds that could be used for a manifold vacuum. In addition whether or not it would have made much difference on the older carbed and MFI motors is debatable.

I your case, I would look at what CR your motor will have and what cams you will be running. Then look at what 911 motors from the carb/MFI years were similar, then select a timing graph to suit.

Then of course that could/should be fine tuned when driving and/or on the dyno.