Oh, i always assumed that the tilt axis is the horizontal center of the screen. When you describe it as a valid solution, it sounds as if the axis is the viewers head. That would be actually a better solution than up/down, with the screen remaining perpendicular.
I was wondering how you guys haven´t yet tried the “Tilt”-button in the vorpX-menu yet and would have given you the idea of trying that one, if Ralf wouldn´t have done it already.
The probably only “downside” of “tilting” the screen up or down would be that it ain´t moving in a “flat” direct way up/down and more in the way of like moving along a sphere, but this also has its good reason, since that way the app makes sure you´re looking at it from the “best” perspective.