[Al Skierkiewicz]

Mark showed me the response from Nasson and it appears to be more of a calibration of the switch actuator than actual pressure calibration. If you move to far in one direction the switch will turn off and too far in the other direction and it will never turn on. The effect is the pressure on the actuator caused by the spring in the switch I am guessing. The manufacturer states that this is only 2 psi. If you want your switch to operate reliably, I would not recommend any adjustment in addition to my previous statements.
Thanks to Mark for this in depth research.