Ok Moony, you and MartinZ eventually convinced me. 
Videos and papers helped a lot.
So now I’m again getting hands dirty with my “default” shader, and, unless someone comes with proper math I propose this node setup:
Instead of eyeballing with curves and arbitrary adjustements, what I did is use fresnel (Schlick’s approximation) to drive the roughness sharpening. Just guessing here: I thought that maybe the same nature’s law apply here as well…
Reproducing a hallway like in Thea video gives me a nice result though.
