Steve, why set your camera levels to 20dB below full scale with a 0dBu output from the mix board? I would have thought that they should be set to 0dB?

The meters on most mixers etc are VU meters while the meters on most (though not all) cameras are Peak Power meters (PPM). They have different calibrations and ballistics. SMPTE says that to align the two meters, when fed a steady tone, 0VU on a VU meter should correspond to a -20dB on a PPM. (The EBU standard calls for -18dB.) When metering a digital signal, 0dBFS (dB Full Scale) is when you "run out of bits" and any higher value is lost and you have clipping. Unlike an analog signal where there can be considerable headroom above 0VU, there is none, zippo, zilch, zero headroom above 0VU with digital for that reason. By going down to -20dBFS for the average level you have a lot of room to absorb peaks and transients that are so fast they might not even register on a VU meter. Later in the post process when levels are under full control you might go up to compressing the average level at something more like -12dBFS with a brickwall limiter at -0.3dB during the final mastering stages. For final release most broadcasters and the networks specify average levels at -20dB and want tone at -20 along with bars for about a minute at the head of the tape to calibrate their playback equipment. I think that's also the Redbook spec for music CDs though I could be mistaken.

At least this is how I think it works ... if I've screwed up the explanation I invite our real experts like Ty or DSE to correct me

Thanks very much for that detailed explanation. I have an upcoming graduation shoot where I may (for the first time) tap the audio board to my camera. This will save me from totally blowing out my audio!