turret rotation accuracy problem

I've already tryed my luck in the unity forum, but I haven't found anyone who could help me, so please excuse the crossposting.

I'm currently trying to setup a turret that rotates and fires at the players mouse position:

As you can see here it's very inaccurate, I think it has to do with the bullet height (y), because with setting y to zero the bullet always hits the center of the crosshair.

I think the bullet-height must be added to the rotation calculation somehow.

 var bullet : Rigidbody;
 
 function Update(){
     var mousePos : Vector3 = Vector3(Input.mousePosition.x,Input.mousePosition.y,Camera.main.transform.position.y);
     var worldPos : Vector3 = Camera.main.ScreenToWorldPoint(mousePos);
     iTween.LookUpdate(gameObject,{"looktarget":worldPos,"time":2,"axis":"y"});
     
     if(Input.GetMouseButtonDown(0)){
         var clone : Rigidbody = Instantiate(bullet, Vector3(transform.position.x,1.5,transform.position.z),transform.rotation);
         clone.velocity = transform.TransformDirection (Vector3.forward * 10);
     }
 }

Anyone could point me in the right direction?