I learned on my 221 Featherweight that when I started sewing on it, I put the seam allowance at 1/4 inch mark. This machine has really helped me so much I cannot tell you. I love everything about my new (old) machine. For straight stitches this is the one I use and I very rarely have any problems. For my fancier stitches, I use my brother machine.
Good luck with your Big block jelly log pattern.