matching seams
I too become frustrated when the corners are off. This is what I do after sewing the piece seam; I keep a small Dritz black ruler next to my machine. It has a moveable red marker on it. I move the to size needed. If the seam is off, then I sew it again. then I move on to careful joining...looking down the seam to make sure the length of the seam if lined up beyond the end (because you sew 1/4" below the edge).