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.