As this thread is a better fit in the ‘Technical’ section, it has been moved accordingly.
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A ½” Hg variance in power valves opening as measured to their rated value would be normal. As a general rule, I do find that new Holley brand power valves open very close to their rated values. A 2-3” Hg difference between the actual opening value and the rated value could pose a problem on an engine that is by design running at a lower than normal idling intake vacuum number. Kudos to you for checking the power valves before installation as that does help to eliminate one more problem that could potentially crop up if not done so. If you find a particular power valve manufacturer that is consistently having a discrepancy in their power valve opening values, then dropping them a note about the problem may help to get that resolved for the rest of us down the road that happen to use those particular valves.
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I’ll add that as the Holley style power valves age, the rubber stiffens which in turn alters the opening rate and can get so stiff that they don’t close at all. Once the power valves are about ten years old, they should be changed out for a new ones.
Lorena, Texas (South of Waco)