Lookat via Single Axis

OK,

I've been digging around and trying to have a single Bone in a models armature to rotate on a single axis to make the armature look at something.

The problem I'm having is that it rotates in a weird ways with each method I've tired.

Here is my current try. https://www.dropbox.com/s/fs9y8mfg6gxw08t/web.rar

This is the closest I've gotten to make it work, but still does not behave correctly.

Here is the code:

 void CalculateAimPosition(Vector3 targetPos) {
     Vector3 aimPoint = new Vector3(targetPos.x - transform.position.x + _aimError, 
                                    targetPos.y - transform.position.y + _aimError, 
                                    targetPos.z - transform.position.z + _aimError);
     Quaternion newRot = Quaternion.LookRotation(aimPoint);
     
     horizantalPivet.transform.rotation = Quaternion.Slerp(horizantalPivet.transform.rotation, newRot, Time.deltaTime * 2.0f);
 }

I've tired The following: http://answers.unity3d.com/questions/36255/lookat-to-only-rotate-on-y-axis-how.html http://answers.unity3d.com/questions/304481/rotate-object-on-one-axis-with-slerp.html http://forum.unity3d.com/threads/36377-transform.LookAt-or-Quaternion.LookRotation-on-1-axis-only

I think the problem is the the fact the bone already has a rotation to it. The default rotation of it is X:0 Y:90 Z:0.