No no no! Those level controls in the mixer screen are the equivalent of the track faders for inputs... and just like the track faders, they DO NOT affect the recording levels. They alter the amount of input signal going to the stereo bus (if they are assigned to it)... again just like the track faders which alter the amount of track playback signal going to the stereo bus. The stereo bus is where all the signals are mixed and sent to the stereo outputs; it plays no part in the recording in multitrack mode.
So the only controls (apart from EQ & FX) which affect the recording levels in multitrack mode are the source trim knobs.
You're correct in that the source trims do not go down to zero - their minimum position is for line(high)-level signals, so if you still get overloads you'll need to reduce the signal externally with a pad/attenuator or mic placement etc.
Marius - if you meant 'unity gain' where there is no attenuation and no amplification (like the 'U' setting Mackie use on their mixer knobs), I think it's about the 1 o'clock position from some tests I did ages ago, but can't remember how I verified it so don't take this as gospel.