As a addendum to my post above about gain, you can just get a feel for things with experience, but knowing the specs of your gear and some general understanding of sound pressure can help a lot.
For example, Gefell m200 cardiods have a mic sensitivity of 12mv/Pa, or about -36dbu. The Mixpre-6 can accept a mic signal of +14dbu without leading to clipping (going over 0dbFS) the A/D. A Pascal (Pa) is a sound pressure of 94db. If I conservatively assume a concert will get to 130db (36db above 1 Pascal), my mics will put out 0dbu (130-94+ (-36)). If I add 12db of gain before the show, I can still have 2db of headroom even if the concert gets to 130db. If the concert only hits a max SPL of 100db, my recording levels will be at -32dbFS, based on +14dbu max input on the MP6. That is, -36dbu for the mic sensitvity at 1 Pa (94 db), plus 6db for going to 100db (above the 94db reference), plus the 12db of gain I set, leading to a total of -18dbu going to the A/D stage, which needs +14dbu to get to 0dbFS. So my peaks are at -32dbFS.
Which seems low, but given the wide dynamic range of the MP6 and the very low noise at -130dbV, the noise of the concert environment is much higher than the internal noise of the recorder, so I can boost 32db in post and everything will be great.
Plus if you really want more insurance after giving yourself plenty of expected headroom, you can set up the limiters of the Mixpre-6, which are supposed to be excellent. Though if you need the limiters since the concert was louder than 130db, you won't be able to hear anything anyway.
Edit: I'm making some quick assumptions above between the max mic input and how that relates to the max input level to the A/D on the MP6. The mic input has a minimum gain of 6db and the max mic input of +14dbu may take that into account, so the max input to the A/D stage may be +20dbu, which would make my calcs above off by 6db, so that 12db of gain at a 100db SPL would lead to -38dbFS peaks on the recording. Still plenty high enough to insure you get adequate levels. Realistically, I will probably set my gain at 18db and will engage the limiters for extra safety.