Accurate Object

I'm trying to create real precise and accurate object selection using ray casts but I can't manage to put the crosshairs in the exact center of the screen. Is there anyway to easily place the crosshair in the exact center of the screen? In the UI editor the resolution is at 1024x768. The crosshair is 16x16. I placed it at 520x392 and it looks centered in the UI editor but it always moves down when I go to play the game.