I use 12 to 15 stitches to the inch, and for a seam guide there are some new feet outhere that have little guides up one side just make sure it is truly a sacant 1/4 inch. I do not like the plastic little foot as it seems to drag against my fabric. I prefer metal. The seam guide someone showed works well with your regular presser foot, BUT it needs the screw, and fyi the screw is an odd size and if not the right size can destroy the threads in the bed of your 221 so that you cannot use the feed dog cover plate with a buttonhole attachment. I owned such a 221 for a long time. Just remembered if you can find one a gauge presser foot, vintage foot for singer low shank is absolutely perfect and can be adjusted.
