It doesn't really work like that.
A cutomer will order what they want, and should be able to.
You'll often find a customer will order the same image as a 7x5, 6x8, 8x10 and then maybe as a 16x24 canvas
The same image, many ratios! and so you have to use your judgement to crop them to the respective sizes.
Paul.
www.photographybyriddell.co.uk