Thank you so much ! Indeed I felt like lost in the middle of the sea with just a tiny buoy thrown by Mr Widget...
An other great advantage of this method is that one doesn't have to take the woofer off the box many times in order to tune the vent length: in my case provided it was radius I coudn't use paperboard or something to push forward or backward for thesting: I had to take off the 2214H and this is risky even done carrefully.... So yes excellent !