When choosing your power valve, remember, the higher the number, the earlier / quicker it will open. Also, it does not add fuel when the engine is idling - that is a wives tail. My ATM's both have 10.5 PV in them because my cruise vacuum was in the 12in-hg range. They had a 6in-hg one in them originally, and i had a bad lean spot when transitioning from cruise to a little acceleration. Basically, the 6in-hg PV were taking too long to open because my cruise vac was @ 12 in-hg. You want the enrichment to come in a couple in-hg under what your cruise vac is. Also, the valve is not a switch that is open or closed. It is spring loaded, so there is some open time before the rated number.
Just my 2cents!