Power valve not opening

Thanks. I agree with the "X inches lower than cruise vacuum" selection, definitely not the half-of-idle vacuum which is only for SBC owners who don't understand how the carb works ;) My idle vacuum is 8", cruise is 15".
My carb is set up for a very lean cruise (71 PMJ) with about a 15:1 AFR, and a relatively large PVCR. It's very easy to drop the vacuum below 10.5" during low-speed cruising and on the highway, just by moving a toe on the gas pedal.I actually tried one, but I get too much unneeded enrichment at relatively low power. The 8.5 seems to be a reasonable compromise.