Danny, what a novel concept. Use your ears?
For an engineer, it is good to know electronics and ratio's such as the above scaling info.
It is also good to have some idea of music, too. Structure, etc.
But the best thing to understand is your system - how to make it sound good to your ears. Because you are making artistic decisions during the recording process. If it doesn't sound good to you, you won't be able to make your own decisions.
I (almost) always find it easier to mix loud sounds. It is easier to simply turn a track down rather than massage it louder w/o any of the crunchyness or crunchy peaks.
But if you don't have high-end equipment it is probably better to leave more headroom than stress the hardware you do have. Pro equipment, the kind w/ too many zeros after the dollar sign, are made for running close to the edge. The typical home recorder's equipment isn't so much. The car analogy is good here. A big-engined car and a 4-cylinder " coupe will both hit 100mph. The former will get there smoother and quicker. Or down in the pasture a truck or jeep won't bottom out where your family sedan gets hung up.
hitting -6 dB is a goal, not a god. If that level "crunches" your system, for whatever reason, don't hit it. If you find signals at -20 dB are too hard to get up to a usuable level record louder. Use your ears and the science behind sound will come.