I cut 2.75" width for binding and use about a 3/8" seam. I say "about", because the seam allowance depends on the thickness of the sandwich. I use a longer stitch length for a trial run of about 6" of sewing and then stop and see how the binding folds over. I am aiming for it to reach the line of stitching while having the sandwich "fill out" the binding. Once I see the trial, I can adjust to a slightly narrower or wider seam allowance. If you want to start with a little wider binding, say 3", you can still use this experimental process.