If you go with the Golden Ratio, your sashing could be 1.5" or 2.25". I would probably go with the 1.5" if you are making a more traditional quilt. If you're making a modern quilt, you could make the sashing 3.75" or even make it wider than the block at 9.75" to give you plenty of blank space. It all just depends on the look you're going for (and of course, if you prefer some other measurement for your sashing, there's nothing wrong with that; art is all about innovation and experimentation! Go with what looks good to you).