Put on a business shirt or t-shirt, and stand with your arms in a relaxed position. Find the spot on the shoulder where all the stitching meet (like in the picture above). This point should in the same position as when you measured the shoulder width. Then measure from that point all the way down to your wrist bone. Make sure the measurement tape follows the shape of the arm slightly. Then add +1cm/0.4in if you prefer your shirt cuff to be visible, or add +2cm/0.8in if you prefer slightly longer jacket sleeves.