I would guess that the scan gauge reads MAP which is the opposite of vacuum. It's called Manifold Absolute Pressure. If your ambient pressure is 30 inches then 0 on a vacuum gauge would be maximum maifold pressure. Most here will tell you 80% load is ideal which would work out to around 24 inches of manifold vacuum. What you want to avoid is enough throttle to bring on WOT (wide open throttle) enrichment which lowers you air fuel ratio. Most BSFC maps show full throttle to be less efficient than 80% load or a few inches of vacuum reading. Basically you want to avoid the last few points of throttle just before it is floored to stay out of WOT enrichment. If you don't have precision instrumentation then just avoid the last 10-15% of throttle position.
regards
Mech
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