[MattBearman]

The relevant info from the link above:

* IMPORTANT MESSAGE. PLEASE READ *

Because the sensor inside every individual iPhone/iPod touch could be built slightly differently, to make the level more accurate for your device, you need to calibrate before use.

To calibrate:

1. Find a flat surface.

2. Hold your device upright (in portrait mode and home button at the bottom), put the bottom edge of the device on the flat surface, then press Calibrate button (the one with a target image on it) to calibrate the portrait mode.

3. Turn your device 90 degree clockwise (now it should be in landscape mode and home button on the left hand side), again keep it upright, and put the current bottom edge (long edge) on the flat surface, then press Calibrate button to calibrate the landscape mode.

4. Put the back of the device against the surface, then press Calibration button to calibrate the face-up mode

5. You are ready to go!