GUILayout alignment issue

Hello, I'm currently working on my game menus, and i have a problem. I'm using GUILayout for creating the buttons but i can't figure out how to align the whole thing. I've set the the coordinates of GUILayout.BeginArea to the most centered point i could get, but, on different resolutions, the whole area moves a bit ( right or left, depending on the resolution ). Is there any other way to center this in order to get smoother results ? I've linked some pictures with different resolutions so you can see the actual problem. Also, here's the script of my menu. Maybe i'm doing something wrong.

 var menu_skin : GUISkin;
 
 function OnGUI()
 {
     if (ball_health.game_over == true)
     {
         GUI.skin = menu_skin;
         GUILayout.BeginArea (Rect (Screen.width / 2.8,Screen.height / 2.5,400,500));
         //game over text
         GUILayout.BeginHorizontal();
         GUILayout.Box ("Game Over");
         GUILayout.EndHorizontal();
         
         //restart, main menu buttons
         GUILayout.BeginVertical();
         GUILayout.BeginHorizontal();
         
         if (GUILayout.Button ("Restart"))
         {
             collect_coins.score = 0;
             ball_health.game_over = false;
             Application.LoadLevel ("level_1");
             ball_health.ball_hp = 3;
         }
         
         GUILayout.Button ("Options");
         
         GUILayout.EndHorizontal();
         GUILayout.EndVertical();
         
         //Options, Quit buttons
         GUILayout.BeginVertical();
         GUILayout.BeginHorizontal();
         
         GUILayout.Button ("Main Menu");
         
         if (GUILayout.Button ("Exit Game"))
         {
             Application.Quit();
         }
         GUILayout.EndHorizontal();
         GUILayout.EndVertical();
         GUILayout.EndArea();
     }
 }

menu 640 x 480

menu 1024 x 768

menu 1600 x 900

Thanks for your help!