If you drive it on the street a fair amount then you want to set the power valve so that it comes on at about 1/2 the cruise vacuum level. So for you something in the 7,8 or 10 range should work for you since you cruise at 15 inches.
But if you drag race the truck then you want to make sure that the PV is open when you need it to be. A 8 should work just fine but you really need to double check the vacuum during a run to see where it is. Going down the track you should be at WOT and the PV should be open the entire time, but at the starting line it may or may not be open depending if you foot brake or not.
Vacuum at idle shouldn't really matter for PV selection. If having the PV open at idle causes a problem then you have other issues with your carb.
I agree with Andy's answer. The only thing I would add is that IF you had a WBO2, the best way to set the PV opening is just before the lean spike happens. And the cleaner (leaner) your car (truck) cruises, the sooner that happens so the sooner the PV needs to open to compensate.
If however, your carb is rich in the transitions and cruise, that earlier opening point is giving it more fuel than it really needs or wants. But most guys do not put that fine a point on it. If you have to guess, then you are better off with an earlier than needed opening rather than a later than needed opening.
Our engines are much more tolerant of overly rich conditions than they are of even slightly lean conditions. So, if in doubt and without any A/F info, there is less chance of a problem with the earlier opening.
If you are experiencing no driving problems on the street with the PV you have, making a change will probably not help anything. But at the track, changing to an earlier or later opening may impact the 60'. Only testing will tell.
