Official comment
Adjust the channel trim so the channel meter shows the levels at the first orange LED, then adjust the MASTER level to around the 2 o'clock position. That should be roughly equal in volume level on both the channel and master when the channel fader is fully open with the crossfader to that side.
When you select the channel CUE and master CUE, you can use the HEADPHONE MIXING knob to adjust the blend of the two sources, but they should be the same when the knob is at the 12 o'clock position.