adjust netural position accleromter

I am having problems with iOS accelerometer calibration. i want to adjust the neutral position of the accelerometer when the player hits the adjust button. i use the following code but the code produces unwanted results e.g. when the device is >90 degrees.

function SetNeutral() { tiltzcalibration = Input.acceleration.x; tiltycalibration= Input.acceleration.y; }

function Update() { ..... var acceleration : Vector3f = Input.acceleration;

 acceleration.x -= tiltzcalibration;
 acceleration.y -= tiltycalibration;

 acceleration.Normalize();
 ....

}