You want it to close as the pipe's tuned RPM is starting to hit.
There is no specific setting that will work for everyone!
Tons of different factors come into play, but a good starting point is an extra 1/4 turn from what you have now (assuming your PV is adjusted to stock settings).
Kick it into high gear and lug the engine, give it full throttle so you can feel the engine making the power slowly, feel the dips and hesitations as it gets on the pipe.
Adjust the PV a little bit and see what it does to your powerband, you'll start feeling it getting smoother if you're going the right way and lose power going the other way (or a step/hesitation/dip in power).
Tweak it all the way to 1/8th or even 1/16th turn, because any more than that and you won't be able to tell the difference in adjustments.

Be sure you tune the powervalve AFTER you tune the carb, full throttle runs for extended durations aren't good if you're running lean!