Doh! CaRX tech support duty! Well, it isn't the first time I've been on call for a calcium reactor setup.
Adjusting the secondary pressure is for bubble size on the carbon doser regulator. So, you can start at 6-7 PSI and adjust up in PSI if you want the bubble larger or adjust down if you want it smaller. You use the dial to adjust how many bubbles per second... but that is pretty self-explanatory.
Aquatek, RCA, Fish Gallery, and I believe Aquadome all carry the pH calibration solutions here in Austin.