The manual on page 16 tells you to " lift " the knob to adjust.

In reality you pull it OUT from the panel to expose the colored band, turn to set the pressure and push it back in to lock it.

The AIR light being out indicates there is sufficient pressure on the supply side of the regulator in the 375. Now you need to readjust the regulator on the 375.

Another thing I saw the Miller guy do was, readjust the regulator on the 375 when the cutter was set to 27 amps and trigger pressed. This is to compensate from pressure drop when the cut begins