I also have an old rotator cuff injury and have to be careful. Sit up higher so that your forearms sit flat on your sewing table without your shoulders scrunching. It seems wierd at first, but helps so much. Keep your elbows in as much as possible. When you stick your left elbow out it aggravates your shoulder. Also try hard to not clench your jaw and neck. I tense up too much and have to be very conscious.