I finally found out how to deal with the CMS to end up with the kind of charts d-nice gets on his cals.
Until now it was impossible to me to have a greyscale as flat as shown on his reports, now I do.
Do as usual, adjust greyscale, as flat as possible from 30% up.
You should end up with some roll off or some raise from 20% down. 5% delta is acceptable at 10%, it's not at 20%.
If your 20% isn't flat, put your pattern on screen, read continuous, and use the CMS primaries only
to fix your low end greyscale.
On my case only red needed 1 click to flatten the low end.
Check the whole greyscale again, you should end up with dEuv (with relative Y) lower than 1 everywhere except 20% and lower, that should be under 2.
When your greyscale is right, jump to gamut, and use the CMS secondaries only
to move them where they belong, if needed to.
If you made a change, recheck greyscale, if still allright, you're done, otherwise repeat from the start until everything is perfect.
Note : when I raised Red CMS to +1, I had to put 1 more click on Red High.
Caution : Green CMS must be handled with care or it will screw up everything, I wouldn't even try to tweak it more than 1 click either side if your greyscale is flat enough from 20% upwards.