But it 100% depends on what equipment your using, different mixing desks have different pre amps and setting the audio gain at one position in one desk is completely different to another desk. For example with yamaha mixing desks you wouldnt go above -12 not because thats the volume you want, you do it because thats the volume the pre amp can handle. While most other mixing desks you aim for the loudest sound to just be below the top so that point on the vu where its above the green in the yellow and not going into the red.
The link you linked has a photo of a yamaha desk so I woudlnt be surprised if thats the only thing hes ever mixed on so he sees the -12dB of yamaha desks as a rule on mixing and not a rule just for yamaha.