This article documents a list of techniques for gain staging audio systems.
DAW & Console
Use a test tone to set the gain on the console so the tone equals 0VU and -18dbfs on the DAW. 
Piezo tweeter & test tone method
For this method you will need A piezoelectric tweeter (these are usually under $4) and a oscillator that generates a 400 Hz test tone. 
Internal gain staging
- Solo a microphone input channel set to pre fader listen (PFL)
- Make sure the EQ and other settings on this channel are bypassed or set flat
- Connect the oscillator to that channel.
- Connect the piezoelectric tweeter to an output channel where you can feed the oscillator to it (such as a control room out or headphone out).
- Turn on the oscillator and turn up the preamp gain on the channel until a buzz emanates from the tweeter.This buzz is distortion expressed in harmonic energy.
- Turn the Gain preamp down until it stops (but no further).
- Take note of any peak light indicator values that accommodate the buzz.
- Set the channel fader to unity gain
This channels preamp gain setting on this mixer is done. Duplicate for each microphone input channel.