I might be a strange one, but I can never get the method of sewing a square over the corner to work out. I have to cut the diagonal and sew a piece on and then trim to size. I use my easy angle ruler all the time for blocks like this.
I even drew up a set of square in a square blocks so I'd know what size to cut the inner square. Then I sew slightly oversize triangles on the corners and trim them to the exact size. Only way I can get them to turn out correctly.