I've calculated the headroom of my highlights based on the tutorial on your website.
I'm getting inconsistent results though i.e. sometimes I'm clipping the highlights.
Several times I have done the headroom test and have measured my saturation point to be 13,284 (Green channel after the black point has been subtracted).
On a canon 5D3 I have done a few tests which measure around 3.5 - 3.7 EV headroom so I am using metering for + 3 1/3.
Today I redid the headroom test to try to figure out the problem (my subject was a super bright white painted timber wall in direct sun). I metered +3 1/3 stops and it still was clipped.
Max value in green channel is 13,672. I've also done some other tests where the saturation point is different again.
I'm using fixed ISO100 and WB5500. I've also tried with Uni WB.
Indoor tests seem to work as expected so perhaps flare is influencing things? I'm often shooting outdoors in bright conditions....