Dart falls on it's face.

to make sure it doesn't open at idle, it could be 1'' lower but 2 gives room for variances in rpm due to 'whatever' can sometimes happen.

no that even if you have to high a rating pv it'll only be open during idle, not at cruise going 40 mph er whatever cause the vac will be too high, but the idle can get dirty and through you off on plug color.

the 2'' lower than vac reading is 'in gear' btw, if it had stock convertor I'd start at half of the reading.

You'll just have to play wih it.