Check your manifold vacuum at cruise and set the power valve opening from that, not idle. You'll be able to see with the O2 sensor that the power valve can be set to open much sooner. It doesn't affect idle.
I'm quite you are correct in your application but..... this is the rule i used to set up my 850dp and it works very well.
The Truth About Power Valves Used with Holley Carburetors - Engine Builder Magazine
An accurate vacuum gauge, such as Holley P/N 26-501, should be used
when determining the correct power valve to use. A competition or race
engine which has a long duration high overlap camshaft will have low
manifold vacuum at idle speeds. If the vehicle has a manual
transmission, take the vacuum reading with the engine thoroughly warmed
up and at idle.
If the vehicle is equipped with an automatic transmission, take the
vacuum reading with the engine thoroughly warmed up and idling in gear.
In either case, the power valve selected should be 1/2 the intake
manifold vacuum reading taken. EXAMPLE: 13? Hg vacuum reading divided
by 2 = 6.5 power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAMPLE: 8? Hg
vacuum reading divided by 2 = 4 power valve. Since there is no #4 power
valve you should use a 3.5.
