I know Holley says 1/2 idle vac for an opening point, but my experience is that often causes a hesitation. There is no vac signal to the PV at idle so a higher (earlier) opening point will not affect the ide mixture. I usually run a 9.5 in almost everything I set up. In my Cuda right now, I have a 10.5 hi flow. My idle vac is 9" or so in gear. I can run a 9.5 and even an 8.5 if the weather is right. But by 6.5, I have a stumble at tip in. I am sure that Holley has forgotten more than I know about carbs, but I have dealt with this issue many, many times. If it helps, I get my PVs from O'Reilly's Auto Parts. They are the same price as Jegs, usually have it the next day and there is no shipping.