How can I make a GUI Rect be in the direct middle of the screen?

        public float h = 0;
        public float w = 0;
     
         // Use this for initialization
         void Start () {
            if(h == 0) h = 20; 
                if(w == 0) w = 20;
 
            }
     
     // Update is called once per frame
     void OnGUI () {
         if(GUI.Button (new Rect(Screen.width / 2 - w/2, Screen.height /  2 - h/2, w, h), "X"))
         {
             print("Hey hey");
             
         }

         
     }