Shouldn't this depend on the headroom of you analogue system (and how hard you hit your preamp). Most systems can output 22dBu (or more) , that makes 18dB headroom at +4dBu. So 0 Vu should be at -18dBFS , 0dBu = -4VU should be at -22dBFS?
I could be wrong but this is what I understand of it at the moment
