A simple question but a very complicated answer.
There's no arbitrary setting for the master fader. Instead, you want to set up a reasonable gain structure from the beginning to the end of your system. At each stage you don't want to be adding needless gain that needs to be knocked down later. Instead, once your feeds from a mic up up to line level, you want each possible adjustment in the chain to be at a similar level.
Add to this the fact that some hardware and software calibrates their faders in a VU-style scale (with zero about 3/4 of the way to the top) while others are in a digital "Full Scale" system where zero is at the top and you have even more confusion.
Me? I adjust the first input gain stage so the rest of my adjustments can be +/- from the zero (VU) position, just to keep things in the nicely linear part of the scale.