What I would do is look at the complete waveform in Audacity (or similar) and see where the average level sits. If most of the mix is at -6dB, use that as your threshold. If you want it to pump a little more, make it -8dB etc.
Play the mix through the maximizer, adjust the parameters and listen.
If the music in your mix is already quite compressed, just use the maximizer/limiter to tidy up the peaks. If it’s older less compressed music, you can squish it more.
I also would avoid taking it right up to 0dB. Leave a little headroom, because depending on what you’re going to do with it, converting formats can affect the levels.
It’s a personal thing but I’d leave a bit more (-0.5 to -1dB). -0.1 is not leaving much “safety space”.
The results from converting will vary depending on what you use and the source & destination formats (which is why you need the margin). I’d suggest processing it, then doing a test conversion and looking at the results - load the converted file back in and see if the levels changed, and which way they went.
Use the iPhone/pad to record. GarageBand recognizes USB mixers such as the x1850.
You can multitrack record on your phone and, if your mixer sends midi, you can record your xfader movements and whatever what else you’re using on the mixer.
You can do a little more this way but it takes time. Or just record your main mix and process it. GarageBand has all the basic tools for what you originally asked for.
I try to make sure that I have a good, undistorted recording. Usually there is nothing that I have to do to a good recording, except (in rare occassions) add normalizing.
Also record & save the recording a 320kbps. Good luck bro!
I know he was asking for advice using audacity. Just wanted to open up another view on how to master.
You can record your mix as is (macro) and tweak levels and eqs or you can capture your flow (micro) and tweak eqs, levels and the like on everything individually.
A good example of micro,
You recorded your mix already and wish you could of dropped the bass of outgoing track earlier. Maybe it might of happened a few times during your recording. Wouldn’t you like to know that there is a way to go back fix it?
At the end a good recording will give you a good master. That’s my advice for the op.