I have always called it a stitch guide, but I'm sure there's a real name. As far as calculating the size, I just start by sewing a big "X" and make sure that I center it and either hit the corners or have an even distance from each corner. then I just pick a measurement and just start measuring from the original stitching. I try and sew on both sides of the original stitch line from upper left to lower right, then do 2 in the other direction, and keep alternating. I have the best luck alternating direction and starting from the center & working out.