Trying to understand gain and mixing

Hello,

I've been DJing for most of this year, and started with a Traktor S4. I'm trying to build my skills to the next level, so to speak: no longer using sync (going alright), no phase meters, no BPM display. However, I realized recently that I didn't really know anything about gain, though I heard many references to it here and there.

I've done a lot of reading on it so far, but it's hard to find consistent opinions on how to use it in Traktor. Ideally, I am trying to set up to prepare myself to DJ on CDJs/DJMs, but I don't have those tools to actually work on. I've seen several articles that suggest, for best club sound with an S4, to turn Traktor's internal main to -10db, and some say that on a real mixer, you want to put the main output around 80%. Finally, the few tutorials I've read all say you want to cut gain as much as possible to just keep the peaks in the song out of the red, which so far makes sense. However, when I look at Traktor's analysis data for each song, I notice its autogain is as much as +5.5 db for some songs.

Basically, what I'm trying to establish is this:

Do you treat gain like EQ and almost never turn it above 12 o'clock? Or is it reasonable to have to go that high on some tracks? If you do, will you introduce clipping and distortion?

Is the -10db setting on Traktor roughly equivalent to setting the main to 80% on a normal mixer (whatever that would equate to -- without the hardware to play with I can only guess)?

Do you want to lower the gain so much so that the PA (or even just my computer speakers) can be cranked without distortion? I have noticed that with the main set at -10db in Traktor, I can put my little monitors to 10 without them sounding distorted and awful, although it does get a bit fatiguing.

Is the other purpose of lowering the main level to -10db or anything else so that you CAN raise the gain on quiet tracks without it breaking up and sounding terrible? (And is that what people mean when they refer to "headroom"?)

Finally, for anyone familiar with the S4, how does the main level knob on the controller itself interact with all this? I understand it's independent from the main level knob in the software itself, so should I just leave the knob on the controller at 12 o'clock? Also, are the S4s channel LEDs accurate? I've seen one or two people say that the orange on the channel LEDs is not actually as high as the reds on a real professional mixer, but I don't know if they were just making stuff up or not.

Thanks in advance to anyone willing to help set me straight a little bit, as I definitely feel pretty mixed up by all this.