When you say 0dbFS=99dbC then it means that you have already configured your system such that its max output leads to 99dBC out of your loudspeakers? In other words, you could set the amp to -5 then and use your input device up to full scale?
Then you are mixing (or creating) with the goal of around 79dB out of your loudspeakers?
So that means you are working towards -20dB output from your software, as we then meet this output out of the speakers.
This means that someone that is playing back your mix on a calibrated system at reference level would play it 6 dB too loud?
Sooo.... firstly, yes, in this scenario our "amps"* are set so that 0dBFS = 99dBC if we set the monitor level to 79 in the studio.
This is because in studio monitoring (film & TV) we tend to use Dolby reference of -20 = 85dBC / 0 = 105dB as the starting point for calibration. We refer to this as "85" or "Fader 7". This is a fixed thing for mixing for cinema.
When mixing for home, we have to some extent to decide ourselves what level to use. If we turn out "amp" down 6dB, we refer to this level as 79.
But, it's a common misconception is that 79 (or 85, or whatever) is some kind of target while we mix. We are not directly targeting any particular room SPL when we mix. Mixers tend to have a dialogue level baked in to their brain that they're comfortable working at. So if you give a mixer a room set at 85 they'll end up recording the dialogue 6dB lower than if you set the room at 79. It's mostly done by ear, with perhaps a casual "comfort check" on meters.
Similarly -20dBFS is only a calibration level. It exists to allow gain structure to be checked (and historically it exists for analog tape calibration) such that we can know that what we're playing back is at the same level as it was recoded. Again, it has nothing to do with the mix itself, or any kind of loudness target. All these numbers are just for repeatability of replay gain.
To your last point, ignoring a whole bunch of other factors (e.g. any loudness spec normalisation which is sometimes needed after the mix, dial norm etc) then yes, playing back at "reference" level would mean that something mixed at 79 is playing 6dB louder than it was mixed.
Since you've no way of knowing what level it was mixed at, nor if any gain adjustment was made to normalise it then [sorry to keep repeating this but...] IMHO there's absolutely no such thing as reference level at home. You can only guess at it by ear. Reference level only exists in post-production and cinema (well, OK, maybe there's a few THX discs, or discs where they accidently used the cinema mix - yes that does happen!! - in that case "reference" would be the level it was mixed at - but again, there'd be no way to know for sure.)
*When I say "amps" I'm referring to the entire B-chain line up. We obviously don't go around changing gains on our amps, we have a monitoring controller, room calibration, console center section etc and we apply negative gain somewhere in the chain to achieve the same result.
Sorry that's very long winded, but hope that helps??
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