Clamping local movement

What is the proper approach for clamping a position in local space? I want to clamp the local forward vector so no matter which direction my object is facing it can only move backward between -5 units and 0 units.

   void Update () 
     {
         if (dragging) 
         {
             Vector3 targetPos = transform.localPosition;
             targetPos.z += Input.GetAxis ("Mouse Y");
             targetPos.z = Mathf.Clamp (targetPos.z, -5, 0);
 
             transform.localPosition = Vector3.MoveTowards (transform.localPosition, targetPos, 3 * Time.deltaTime);
         }
     }