I've yet to test it at a concert. But after messing with it for about 20 minutes, I figured out that the gain can be adjusted while recording using the up and down arrows (DPC, A-B buttons). Also, when you start recording, the REC/PAUSE button emits an LED light. This can actually turned off in settings.
My favorite feature so far is how fast it starts up compared to my other recorder, the Roland R-07. The ICD starts up in 1.50 seconds. The R-07 takes 20.50 seconds.
BUT, to answer your question, if you decide internals, just practice with the opening band. I think 4 would suffice. If you have external mics, you would have to figure it out on your own.