I've confused myself.
When mixing down do you adjust the the Master Bus Input Gain or the Master Bus Outptvlm?  Is the only determinant the fact one is pre-FX and the other post?  i.e. with no FX in the Master Bus it makes no difference?
 
 

I always keep my master bus output fader at 0dB
 
Also, I never have to adjust the input gain to the bus because everything is properly gain staged when it gets there.
If you're running other sub-busses to which all of your tracks feed, I'd be inclined to lower them by a set amount per bus IF the signals coming into your master are too hot.
 
What sort of levels are you seeing hit your master?

Cheers for that.
I'm trying to average roughly -18db, ( a good level for the Greg Calbi presets in Ozone7) so I've got the Master Bus input gain pulled down to -5.8. I know, my staging is duff here and I should probably pull all the tracks down but I always seem to ruin a good mix when I attempt to do that (it maybe an aural illusion, I don't know...) I've got no FX on the Master bus and only one (vocals) bus going in along with the individual non-vocal tracks.

Hi. I keep my Master at unity as well.
If I need to adjust the overall level, I just Ctrl drag the Master up or down, and the rest of the buses follows. I then double click on the Master, to set to unity again.
It might not how the pros do it, but I find it easy if I go into the red or too low.
All the best.
 

Here is a remedy to try if you are Ok with the post sound quality of this.
 
Go to pro channel of your master buss >>> engage the Console Emulation (N-Type is my preference for this but experiment to see what might suit you)>>>>crank down the trim on the Console Emulation Module till you get your desired output level on Master Fader meter.