I'll explain why I use -0.2 dBFS:
This is my usual mix chain:
1 - Pultec
2 - Shadow Hills Mastering Compressor/SSL 384/API 2500/Focusrite 330
3 - Crane Song Ibis
4 - Crane Song STC-8
5 - Crane Song HEDD with processing
I like to clip the HEDD a bit on the way in. It actually does a kind of limiting because it rebuilds the clipped peaks so they don't end as flat clips and gives an extra "smack" to the mix.
The HEDD max digital output with the processing on is -0.2dBFS so that's why I use that as my max peak.
There's no headroom but the mastering engineer can lower the volume digitally before hitting is mastering chain if he needs.
My mixes are about 2-5 dBs RMS lower than the final result.
I never agreed in delivering to the mastering engineer mixes that have a dynamic range of 8-12bBs to be squashed. That could make all the bounces that were so hard to achieve go out of whack.