How are Holley power valves rated for a given inches of vacuum number? Is that vacuum when they just start to open, or when it's sufficiently open so that any PVCR up to, say, .080" is not being restricted by the valve seat?

I just put an 8.5" PV in my 950 (71 PMJ and .066" PVCR) and there is no change to the wideband AFR until the vacuum drops to 7-7.5". The AFR doesn't drop all the way (from the 14.5 at cruise down to 12.5) until 6" or less.

My first thought is an inaccurately made (or marked) PV, but the 7.5" that was in there required even less vacuum and was too lean at part-throttle.