If a knitter has made her buttonhole band EXACTLY as written regarding the number of picked-up stitches, and on which side facing, etc...then you can create the buttonholes as written, too.
However, how many of us do that? My buttonhole bands are always longer, or shorter. So I have to improvise. There is no formula, sorry to say.
However this is what I do:
1) I knit the button band first.
2) Based on the ribbing of the button band...I place pins on all of the ribbing locations that will receive a button on the button band.
3) If you decide that the buttons are to be located 'in the ditch'...that is to say, within in the purls...then the buttonholes must also be created within the corresponding "ditches" in the buttonhole band.
4) And that is your pattern "work order" for the buttonhole band.
Whatever style of buttonhole you chose...the "holes" must be exactly where those pins (buttons) are on the corresponding side.
It isn't hard...but it is time-consuming. Don't short-sheet this step. The ribbing must match up when the cardigan is buttoned up or else it will look tragic.