Holley has said for many years that half the idle vacuum is the recommended opening point for the power valve. Why that has never been updated is beyond me.

First off, there is no vacuum signal to the power valve at idle. So, it's relevance to the power valve is questionable. More important is the cruise vacuum. Going from 15" to 6.5" before the PV opens is a long way. Too long in my opinion.

I personally have never seen a PV open too soon on a muscle car, but I certainly have seen them open too late.

So if it were me, I would put a power valve in that opens somewhere between 8.5 and 10.5. No matter what the idle vac is.