Offizieller Kommentar
The volume output of your headphones will depend on the type of headphones, their impedance, and rated output.
My recommendation would be to adjust the channel trim so the meters show peaks around the first orange LED, then with the fader open and the crossfader to that channel, adjust the master knob so the master meter shows the same level. At this point, the volume levels between the master and a channel will be equal.
Now, the only thing that will make a difference in the headphones is the headphone volume itself - and again, that will depend on your headphones and what you find tolerable. I typically mix entirely within my headphones (no monitor) and when I'm at bigger events, I'm wearing my custom moulded earplugs under my headphones. So yeah, it gets loud, but my hearing isn't going to pot!