Holley 4150, Need rebuild help..

You want to check the vacuum at idle with the car in gear if an auto trans. Let's say you get a reading of 10 on the vacuum,you want a powervalve that's half of the reading,so you'll need a 5. If the vacuum pulls the powervalve open sooner than it should if will be rich,or if it is open at idle you'll never be able to tune it right & will be very rich at idle.