If the vacuum is lower the fuel pressure will be higher. You can't work back to work out what the pressure would be at correct vacuum as it isn't a linear adjustment on that regulator. Its 1.7:1, you could try to use that, but doubt it would be reliable info.
The regulator was adjusted on the rolling road. No more pressure can be added because it will overfuel at lower rpm/full load as the boost builds up. It overfuels at high rpm but half throttle as well.
On a boot from idle the a/f ratio goes rich up to 10:1 as the boost increases between 3-4k, then it settles down after that point.
If Ant wants to use an adjustabe regulator he should set it per the factory service manuals suggested fuel pressure, then take it to a good rolling road to see whats happening at different rpms and loads.
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