Here's how I do it...
- Set your channel gain so the channel meter shows peaks at the 0dB mark
- Take the channel meter to full-open
- Disable the crossfader or put it to the side of that channel 100%
- Turn the master knob so the master meter shows peaks at the same 0dB mark
So long as your channel and master meters match, the volume in the headphones for each source will as well.