Calibrating LEVEL in INAV (and other?)

When do you ‘calibrate’ for ‘level’ with INAV or Cleanflight etc for FIXED WING installations. Do you calibrate outside of the airframe on a level surface? The FC may not be installed ‘level to flight’ in a fixed wing. When do you adjust the ‘pitch’ in the FC software, do you use this for ‘fine tuning’?

Calibration can be done anytime, but as the fc needs to adjust to a level horizon, it’s only practical to be installed in the airframe and setting on a level led surface. As for making adjustments, the aircraft has to have made it’s maiden flight and adjustments are made accordingly. Those adjustments can also be made in flight.

My bad, I don’t know about fixed wing.