Is the 5 5/8 piece for backing? I tend to buy my fabric for the front piecing first and only after the top is finished do I buy the back. There Is an app from Robert Kauffman that I use to help calculate how.much and you never know.if you are going to want a 42", 60, or even an 108" wide for your backing. (Yes , I did a 108 on the back of a throw one time because it was precisely the right color. No regrets!)
I have the luxury of a Joann Fabrics and 2 good quilt shops in my vicinity so I can do this and it has worked well for me. I know its not everyone's situation.