This was wrong 45 years ago, and it's still wrong now.
Passing along bad information for decades won't make people any better at tuning.
The power valve doesn't affect anything at idle, unless the valve is defective. If you set the power valve open at half of idle vacuum you will have to crutch the tune up to cover the hole in the fuel curve on tip in.
Set the power valve opening a couple of numbers below cruise vacuum. Mark Whitner posted a great video on you tube of power valve opening at idle and proves the long told tale of half idle vacuum dead wrong.
