On my vintage machines, I use a magnetic seam guide. It works well, but the one you have will work too. I was taught that to get your 1/4" seam you cut 3 2 1/2" squares of fabric and sew them together with a 1/4" seam. Then you measure them. They should measure 4 1/2". If you get a different measurement, you need to adjust your seams to get the correct measurement. This method allow you to get an accurate measurement of the final product no matter what type of thread or fabric you are using.