If your buttons and buttonholes are not positioned correctly, your garment will either be too tight or will gape and the collar will not meet at the CF (I know).
Most sewing and pattern-making books tell you that your buttonholes overlap the CF by 3mm (1/8″) but, unless you have a very wide shank or very distant stitches on your button, that will be far too much. As it turns out, there is a simple trick for centring your button and positioning the overlap so that your garment will not gape.
Positioning your Horizontal Buttonhole
Centre the button over the centre front, put a fabric pen (or a needle with a knotted thread) down the hole that is nearest the fold of the fabric, with the needle nearest the middle of the button (as if the button were looking away from the fold). This mark will be the start of your buttonhole.
The Length of the Buttonhole
The length of buttonhole will be the length of the button + 3mm OR you can wrap a ribbon round the button’s diameter and pin it tight, remove the ribbon, and the folded length + 3mm (1/8″). Measure this distance from the start of the buttonhole (the point you have marked) away from the fold. This is your marked buttonhole.