checking bench vacc motors

[JRM]

Larry, and the other bench building gurus.
I took the top off my bench, attached the incline manometer to both sides of the 1265 orifice plate hole. Ran 2 taps one on the motor side of the plate and one above the plate. made a quicky top with 2 4" holes and turned on all 8 motors. the virt pressure reading above the motors was only 1" below the motors was 10". Now if i take the 1265 ori plate hole and convert it down to 10" it drops to 815.74. If i take my incline reading of 85% and deduct it from the 815.74 it gives me 693.73 @ 10 ". Am I making the correct asumption that If i wanted to get to 28" I would need to add another motor.