Realtime Object Rotation on iPad

I have an box, and I would like to reflect real gravity on the box.

There are numerous items contained in the box and I would like to rotate my iPad to affect the rotation of the box.

I cant get this to work quite right.

Here's the script I'm using (JS):

 function Update () {
     rotation=-Input.acceleration.y;
     var rotationVector:Vector3 = new Vector3(0, 0,rotation);
     // Move object
     root.transform.Rotate(rotationVector);
 }

root is the gameObject of my box.

The rotation on this seems to be quite erratic, I can't get it to work quite right. Basically, if my iPad is Portrait (It's a landscape-right app), I want the box to rotate to 90 degrees; this means it's be -90 the other way and 180 if viewing in landscape-left. I know I can detect orientation directly, but I want to detect for all values in between and not just fixed values.

Can anybody help?