This^^^A more technically precise method requires equipment not usually available to the hobbiest.
But if you happened to be so lucky as to know a place with NOx gas measurement and a load dyno then it can be done by observing the combustion.
"The purpose of the power valve selection is to enrichen the mixture prior to the combustion temperatures going too high. To set a powervalve correctly requires a NOx sensor. Different engine combinations need fuel enrichment at different power levels."
Bruce "Shrinker" Robertson. post #4 "Another Power Valve Question" archived Innovate Motorsports forum
And this^^^ou're welcome to your opinion, and I'm welcome to mine. The engine needs to go rich when the load increase gets the combustion so hot that NOx production shoots up. If we want to find a relationship between more aggressive cams and vacuum at idle, we can find plenty of data that shows more aggressive cams tend to lower idle at vacuum. This would result in selecting a lower power valve opening when in fact most hot rod/race engines will need enrichment at less throttle and higher vacuum.