Thanks for your answers. It sounds like you're telling me that I can only center one object on a wall with the centering tool. If by "dimensions" you mean, selecting the window and moving it until the dimensions on each side of the window are equal, I have tried that and it is, at least for me, nearly impossible to get it exact. It seems to move in a jerky fashion and it moves either too far or not enough. Is there some other way to "dimension" it?