The best advice I've ever read is to make the button hole fit the button. Adjust the hole to fit as you're knitting it, either adding stitches or subtracting. Push the button through to make sure it's not too loose or tight before you move on to the next hole. Life is too short to be driving around town trying to find the right size button to fit that particular size hole the pattern wants you to do. I have better things to do with my time and a perfectly good stash of free buttons in my sewing drawer.
Welsh corgis are cool.