The knot is obviously your limiting factor. Also, what the opposite side of the walnut board looks like, and what you need to avoid. A buddy and I made a couple of plexiglass blank patterns to avoid cracks and knots while laying out blanks. Also, making sure there is no center pith in a blank. If the board contains the pith, you will need to avoid it in the blanks by being on each side of it (which might mean you only get two. Or two fulls, and some half stocks).
And finally, having relatively straight grain through the wrist area is important. If you’re cutting these for resale, per Dennis’s comment, they need to be of a universal common size. You may want to get a pattern from an existing stock blank (and be able to take a 44” or 46” barrel length) that anything can be made from. If you’re making something for yourself, not as important. But certainly worth capturing the burled wavy grain in all three buttstocks, even if it means squeezing them a bit.