Here's a thread where we discussed the diagonal seam, has a picture and links to Flynn's calculation page.
https://www.quiltingboard.com/vbulle...p-t325542.html
This is best when you have a piece of fabric that is extra long, but not quite wide enough for your back. You cut on the diagonal and slide the pieces so you have a wider (but shorter) piece. Is cool geometry/area/math stuff
