You said you're a little short of a 25" square. How short are you? How big is the quilt? If you're using straight of grain binding, then joining the pieces on the diagonal uses a little more fabric than Homespun's calculations. Straight and bias binding both require exactly the same amount of fabric going into the quilt, but there are differences in how much waste is produced. I think there is more waste doing the straight binding if you are also doing diagonal joins.