""I am by no means challenging you but I was taught and have read to do the opposite IE: set the power valve 1" or so below the lowest vacuum reading in gear. The reasoning was that the power valve could / would otherwise open when idling in gear casing an overly rich mixture. So could you explain the theory / reasoning behind your method ?""

I will give it a shot.
Can the PV be open at idle? Yes, IF the throttle blades are open enough at idle to get the main circuit started. But the OP said that his metering screws will shut down the engine if they are turned all the way in. So this means that he is still on the idle circuit and not high enough on rpms to get the carb transitioning into the mains. So his PV selection should not be affecting at idle.