Mixing and Mastering for QUIETness

Tons of talk all over the internet about mastering for loudness; how to get your track to compete in the loudness wars without sounding like crap, yadayadayada. I'm 1000% not a mastering engineer, but still master my own work. All I really do to that end is try to mix to around -6db, and then 'master' with a simple eq and limiter to bring it up to just below 0db.

Lately I've worked on a couple of pieces that ultimately are intended to be on the quiet side. What sort of specs are you supposed to hit for that? How quiet is too quiet? The only reference that immediately comes to mind are most recordings of Gustav Holst's - The Planets. Mars is massive, while pieces like Venus and Neptune are super mellow. I haven't brought them into a session for a reference (I really need to do that!) but what are the common 'rules' of engagement here? Seems like when i force my quieter pieces up to 'normal' spec, it's just too much. But I also don't want them to be too quiet and disjointed from the rest of my work (especially on the same album).