You're right, the PV dumps fuel into the boosters when it opens at it's rating number and there's air rushing through the boosters. But when a PV blows the diaphragm gets ruptured and now fuel is being sucked through that rupture into the manifold through the same vacuum port that supplies the PV chamber, causing an over rich idle problem.
Back to the OP's question, you need to use a vacuum gauge to determine the PV rating you need. Hook it up, go for a ride and do a few easy throttle accelerations, don't fully mash the pedal to the floor cause your vacuum will drop to zero. Watch the vacuum gauge and notice what it drops to. If there's no hesitation your PV is most likely the right size, if there is any hesitation you'll want a PV with a higher rating than you now have. (if you have a 6.5 and there's hesitation go to a 7.5 and test again.)
I'm not trying to bash Dave, He's a very bright guy and very helpful on here.