Not having hardware to test on is a bit of a *****, so I've gone off the wiki page - and, more particularly on the raw values it says are returned.

Does it rotate the correct way as you move the device and "flip" when you stand the device on its side? Or is it just rotating the wrong way all the way round?

On calibration, I was thinking about storing the values returned when a "Calibration" menu option is selected, and subtracting them from the current values (use case would be to take into account any non-vertical mounting in your vehicle). But I'm unsure how well that'd work from the maths point-of-view. However, danielwilms mentioned it was reporting -4 deg on his table, so there may need to be some system-level calibration of the accelerometers as well.

I was aware of the other app called "Horizon" but a) it's unmaintained (AFAICT); b) not in Extras. Suggestions for other names for my Horizon welcome :-)