Also look to your 1/4" foot and single hole faceplate for guidance. On my Bernina my 1/4" foot is sized for a 1/4" seam width. The faceplate has guidance marks for lining up 1/4" seams. Plus, the plastic tray that fits over the base to give me a wider base to sew on also has markings to line up for 1/4" seams. Using all these I can sew half-square triangles with only one drawn line from corner to corner and sew accurate seams.