I'm about to sew a cover for my ottoman with fitted corners (not pleated). I saw the Martha Stewart video but I couldn't figure out how she knew to cut 5" for the corners. She doesn't discuss how she figured that out.

Does anyone have suggestions on how to cut out the fabric for the corners? How do you know how much to cut away?

Here's the video I referenced: http://www.marthastewart.com/1005656/how-make-ottoman-slip-cover