This is a simple Level Gauge. But it's accurate! 0.001 degree accuracy.
Even a paper thickness influence to the measured level.

Full version and Lite version are released, separatedly.
Lite version doesn't support detail adjustment function.
However, rough calibration and the measure accuracy are same with full version.
Full version also support Zero function that makes the current condition as horizontal, to compare distortion.

Howto calibrate:
Step1: Put you iPhone at a flat place and repeat 4 times for 4 angles (vertical up/down, landscape left/right).
The place should be as flat as possible.
Step2: Push upper 'Set' button of calibration for each angle.
Step3: After setting all buttons, calibration is automatically done.