It's the point where it is supposed to start opening. They can be off a bit.
There is usually some lag between vacuum reading and observed AFR changes. Shouldn't be much though.
Starts closing, it’s open till the vacuum is less than the # , say 6.5, below it’s open, above closed, no middle ground. Nothing more than a on-off switch. The PV is a valve has no bering on how much fuel flows thru it, holes are bigger than the restriction.
It's a spring, so a 7.5 valve is fully open at 7.499 inches? Not a chance. They don't open like a light switch.
Maybe because the vacuum runs past the opening point when into a WOT situation, it seems as an on/off deal.
Normal cruise conditions the PV should be closed. Engine off, yes it should be open. Idle, could be either open or closed (should not be on main circuit at all).