To keep things simple, I would recommend -6 db, because it gives you a fair amount of
"headroom" ( the space between your signal and 0db) "just in case" you get "inspired".
In mixdown, you can push the signal closer to 0db ( about 1-2 db away from 0 is good) and it will still sound good.
If you tried this method on tape, your signal would be so low as to be basically unusable-analog always allowed for a bit of "over recording" that is, you could record "hotter", and, as long as you didn't distort, you could push as close as you dared to 0db.
The cool thing with Digital, is that the "push" occurs in mixdown , using volume control and compression. Try recording "hot" with digital and you're in for a very rude surprise!