Hi Herman, Bryan, thanks for your quick answer,
As you said, this tool must have an overpressure escape valve if intermediate pressure is increasing - first stage seat doesn't seal- as well as a purge valve - because there his no other way to purge it (except emptying your tank... of course...)
These 2 constraints convert this tool into something far more complicated than just an intermediate pressure gauge that you clamp the hookah port and therefor probably not worth to manufacture for a small market.
On the other hand, reading the Traditional Method (thanks Bryan for the link!! - I'm French and blond.... would never have found that myself...

) it looks quite straight forward to finely tune the intermediate pressure and get a correct result.
So many thanks guys - as always - for your much appreciate help, and even if I would have loved to check the intermediate pressure of my DH, I will use the traditional way to tweak them.
Nice Sunday evening to you all.
Cheers
Stéphane