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 ButtonholeCentre 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 ButtonholeThe 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.
Marking the rest of the buttonholes
The rest of the buttonholes will start the same distance from the CF and be the same length. One will be at a button's length from the top, one will be on the bust-line, one at the waist. The rest are distributed evenly in between. It is easiest to lay the buttons on the pattern or garment and arrange them as well as possible. Then space them as evenly as possible.
If you want a set number of buttons you can find the space between the buttonholes like this: length from top buttonhole to bottom buttonhole) divided by (number of buttons - 1).
E.g. if you have 36cm from the top buttonhole to the bottom one, and you want 10 buttons, divide 36cm by (10-1 = 9) = 4cm between buttonholes.
This is a lot of buttonholes for such a length, but it works as an example because of the simple maths.
I working this out from instructions in the chapter about facings in Patternmaking for Fashion Design by Helen Joseph Armstrong.