I have also found that you want to check what the tile configuration will be around the toilet. You don't want to have to cut thin slivers of tile, or a three-quarters inner crescent around the flange. As Mike suggests above, get the two perpendicular lines in the "center" of the bathroom, but check edges and around toilet flange, vanity, any other built-in to make sure you don't have a tiny piece at the edge. Move the "center" point over to increase the small piece or eliminate it completely.