My PT friend had me put my sewing machine low and my chair high. Never hunch shoulders or jut out your neck to sew. You should be able to just tilt your head down. Back straight. I can sew for hours with no problems once I made this posture a habit. I remember sitting this way in school when posture was important and taught. The shorter kids had cushions to be at proper table height. I don't think kids are being taught proper posture in school now.
Oh if you are in a workshop try stacking two chairs to be at a better height.