If your top is 45 X 45 you will either have to have a seam or corner blocks. You can add the 45 inches to the two sides but when you cut for the other sides they have to go over the 2 sashes you've already sewed on. Does that make sense? I usually plan to cut the borders without seams but I do like the look of a block from the quilt in the border in some instances. This can also be used if you don't have enough fabric to cut without seams, make a deliberate insertion in the borders as an added touch.