Acceleration not parallel to the ground.

Hi, im making a game using the accelerometer to control the character (x,y) like a plataformer, wich works very well, but i find the acceleration parallel to the ground unconfortable becouse the player will have to always look straight to the ground, otherwise the game character will reach more speed if he changes the phone angle for a more confortable vision (which is very critical to my game since the character can't touch anything or it will be destroied).

I want to know how to modify where the device orientation angle starts, so the player won't have a neck cracking :).

Something like a calibration where the player feels more confortable to play, and reset the device orientation to that position.