After a bit of digging through the E90 forums I can know understand how this works - uses the accelerometer - the sensor that detects the phone being spun from potrait to landscape.
The guys on there seem to be getting "fairly" accuarate results with the BMW's but all seem to need a few shots at different positions of the phone in the car and different settings (weight mainly).
I'm beginning to come round to the idea now!! Perhaps you should give it another bash Ben?
Readings are seeming a bit more accurate now. Have you tried sticking it flat on the we cubby hole to the right of the steering wheel, above the adjuster the the light beam level? That could be ideal for keeping it flat.