For recording live music, 20db gain seemed to be the sweet spot for many of the old school fixed gain preamps like the Oade M148. So that's where I've preset the gain in my Advance mode presets (this is the 'gain' setting in the advance mode menu). At the start of a show I position my faders straight up to 12noon and then adjust from there once the show starts. Seems to work out pretty well. I don't need to move the faders too much from 12noon during the show and my meters are peaking in the -12db range with good, but not ridiculous ear splitting, volume. I never record in 16bit anymore, so I've been using the -12db peak meter reference for many years now. I'm OK with -12db peaks because I'm not worried about the noise floor since I'm recording live music (so, you never hear any self noise anyway), and by keeping the headroom with peaks at -12db, I never need to worry about 'overs', which is great because I don't like to rely on limiters. Obviously, this requires me to bump levels in post, but I'm totally OK doing that, since I'm recording in 24bit and these mixpres have such a low noise floor.
I actually prefer running the Mixpre-3 in custom mode and use the front knobs as gain controls rather than faders. In Custom mode, go to the Custom setup menu and select Basic under the Gain setting. Again, at the beginning of a show, I'll set the gain around 20db on each of the gain knobs and generally get peaks around -12db on the meter during a loud show without having to do much tweaking.
Hope this helps.