Once I got in the habit of setting my needle at the same number and using the outside edge of the presser foot for a guide I was much more consistent in my seam width. I usually sew a 'scant' 1/4" and as long as I stay at that width I come out okay. One of the things I try to remember is whatever width seam you start with - finish with. Also, your pressing can be a problem, not getting seams flat, or stretching or whatever. I use a lot of starch and try to remember to PRESS, not IRON, before trimming my blocks.