Theres 2 new expressions in the new beta update (r143) which are ScreenWidth and ScreenHeight I haven't experimented with it so I'm not sure if the windows hotspot is top left or center but try these:
For Top-Left Hotspot: Set Window.X to (ScreenWidth / 2) - (WindowWidth / 2)
For Center Hotspot: Set Window.X to ScreenWidth / 2
Then repeat for Y/Height