Your settings for left and right gain will depend on the band and the venue and will differ for each show. The idea, in my opinion, is to get as large a gain as possible without clipping. Since the decibel range will vary throughout the show you will need to set it so that it doesn't clip over the range of the show (and I would not continually change the gain during the show...that just sounds bad to me), so I set mine to average at about -6 and have found that for my mics and recorder that works pretty well.......but like others have said, this will be a trial and error for you. Also since my mics are not matched (or don't seem to be), I set the levels for each of mine slightly different.
Until you go out there and do it, you'll never know. I realize that you want everything to go perfectly and don't want a to blow a recording, but that's the only way to get to know your equipment. Good luck!!