2.65 kg/100km of CNG over 290 km.
(NEDC - the EU's EPA value - is 2.9 kg/100km)
Not too shabby.
There was still room for improvement, as I lost a bit here and there, even got stuck in a traffic jam.
Done on a mix of 3/4 High caloric CNG and 1/4 Low caloric CNG - meaning I lost a bit there as well.
As a reference :
Gerhard Plattner used 2.39 kg/100km (using only High caloric CNG) on his record trip in a nearly identical twin of my car.
That's only 10% less than what I was using.
I guess I have my next target ...