You need a constant reference point so you disconnect it. With it attached the engine vacuum will vary from car to car and at different engine speeds changing the fp. You set it up with the line off at the chip burners rec and it should increase one psi per psi of boost. If you go + or - the baseline you will get a nearly linear increase or decrease in fuel delivery across the board if your BLM is locked at 128 (no correction) at WOT. PM me if you want more info and ill explain it further.