Measuring a button for a buttonhole

Have you ever picked out a big juicy button for a sewing project? 

Have you then only measured the diameter of the button and ended up with a button hole that is too small for your chunky button? 

Fear not! There is a way to work out exactly how wide you need to make your button hole for chunky fat buttons. 
I recently read this technique in Couture Sewing techniques by Claire B. Shaeffer