I use the same method a quiltingcrazie, happy sewing!!

If you are going to preshrink, be sure yo get enough after shrinking and squaring. Cotton will typically shrink 3-5%. I recently preshrunk a 108" back and I lost 5". That is close to 5%. For the king, I'd look for the 120" wide and then preshrink.

JoAnn's does have very wide fabric for backing. perhaps you could use a coupon. then perhaps no piecing if it is 108 inches wide.

I would go with the extra wide fabric provided it's wide enough with the 4 extra inches you'll need per side to machine quilt it.