This is for a 440 six pack 4 speed car. I have 15" of vacuum on my gauge. I currently am using a 6.5 power valve. What is the rule of thumb to determine what size power valve is needed.

On your deal ( 4 speed car) you could try a 8.5 to see if you like it better than the 6.5, it will open sooner at light part throttle once the manifold vacuum drops below 8.5 inches versus dropping down to 6.5 inches to make it add fuel to the carb. like it does now

Holley has said for many years that half the idle vacuum is the recommended opening point for the power valve. Why that has never been updated is beyond me.

First off, there is no vacuum signal to the power valve at idle. So, it's relevance to the power valve is questionable. More important is the cruise vacuum. Going from 15" to 6.5" before the PV opens is a long way. Too long in my opinion.

I personally have never seen a PV open too soon on a muscle car, but I certainly have seen them open too late.

So if it were me, I would put a power valve in that opens somewhere between 8.5 and 10.5. No matter what the idle vac is.

All the people I know, a lot of them, that had problems with the power valve opening while in gear at idle had the primary throtte plates adjusted so they where on the transition circuits, not on the idle circuit only
Been there, done that, more than once

The correct way to choose a PV is using an AF meter and driving the car.

It's really easy to do if you have a meter that can log. Use the RPM, AF and vacuum readings to get it right.

Ther are generalizations for selecting, yet, only the engine and car will tell you which is the best.

No vacuum signal to the power valve at idle, sorry but that is INCORRECT. The power valve has full manifold vacuum to it at idle. If it didn't it would be open. The power valve gets it's vacuum from a port in the baseplate, not the carb boosters. Once the manifold vacuum drops below the power valve rating the spring in the valves opens it. Just look at the chamber in the carb for the power valve vacuum side and notice the port at the bottom of that chamber and where it goes, it goes directly to the baseplate and subjected to the manifold vacuum.

Problem is the PV dumps fuel into the main well. Boosters should NOT see enough signal at idle to drag in fuel. I think Dave may have mixed things up a little bit. He's one of the sharper guys around here

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.

Diaphram blowing is a different issue for sure.

I have a vacuum gauge hooked up and says 15". I'll drive it next week and feel for any hesitation. I had my center carb apart due to a stripped accelerator pump screw and was just wondering about the right power valve.

Just tuned a set for my 340 and 6.5 pv is what worked best with a 66 main jet. mild 340 and now runs perfect!

A bit out of my area (so FWIW) but when dealing with "hesitation" I think you'd want the A/P to be correct also, immediate tip in/correct nozzle/correct pump cam (holley).

Definitely one of my clumsiest responses to say that there is no vacuum to the PV at idle. I oversimplified to the point of being incorrect.

Sorry about that.

The point that I was trying to make was that the cruise vac is usually more relevant to the PV opening than the idle vac. And that it is better to have a slightly early opening than a slightly late opening.