Been seeing a lot of new producers asking why they should keep their DAW master meters around -6dBFS (or -3dBFS, -4dBFS, etc). While a lot of mastering engineers are the ones asking for mixdowns this way, there seems to be some confusion that this is something that is needed for mastering. In truth, mastering engineers could care less what the headroom of your tracks is, as long as there is some.
The real reason you want to try and keep some headroom in your mix downs is to make sure that you’re not inadvertently clipping. A lot of synths and effects use random modulations, even some dynamics processors are designed to mimic analog processors and thus might have slight variations each time you process audio through them.
By aiming to keep roughly 6dB of headroom on the master channel as you do your mix, you’re just ensuring that some of this randomness doesn’t clip. Just because your mix peaks at exactly -1dBFS one time when you play the song, doesn’t mean it will peak to that same value each and every time. Leaving some headroom just eliminates having to worry about this, it’s a safety net, nothing more.
In a perfect world if you’re 100% sure your mix is not clipping, you can render it as close to 0dBFS as you want. But that doesn’t mean that you’re going to gain anything by doing so, your mix won’t sound better. So there’s really no need to push things that hard and risk clipping your file permanently.
As you can see, I tend to tell people to aim for roughly -6dBFS, but that’s just a personal preference. Some people recommend -3dBFS as well, either will work fine, just be sure you’re also working at 24bit or higher.
So what if your mix is too hot, and you don’t know the best way to fix it? In general I tell people it’s best to get in the habit of leaving the master fader at 0 and just lowering all the tracks by the same amount until you have the headroom you want. But there’s really nothing wrong with just lowering the master fader too, if that works better because you have a lot of track volume automation for instance. Use whatever is easiest for you, the key is just to get that safety in place.
I hope that helps clear some of this up, let me know if you have any questions!