Im sorry I dont have one in front of me, so am only suggesting based on how most work...
The adjustment is inside the dosing chamber - if there is a screw on the top of the central part, then thats it.
Dont make too much of an adjustment - test as you go, and if you go too far, youll unscrew the whole lot. ;D
Be aware that these measured doses really only work when you have a dosing chamber that has a LOT of ground coffee in it, and that of course, is something any coffeesnob will frown upon.
I suggest you continue using it as is - after all, a few extra thwacks are all part of the theatre and fun of coffee preparation! ;)