Ye just shift the datum to suit the best lay out.Thanks Dave, I will use centre of the window width wise, but I think I will have to raise the height of the bottom tile to the floor and have a smaller gap for the top tile to the ceiling. 90 degrees to the left Wall is a 40mm high shower tray, I don't want a slither of a tile resting on top of the tray. Anyone have any thoughts on what is minimum height as first tile above a shower tray that is 1.7m long ?