I made 4 queen or larger quilts this past year and I used a piece back for each quilt. I used fabric 43 inches wide down the center length wise, then a used a coordinating fabric (usually a solid) for a strip on each side of the 43 inches then another piece of the center fabric on the outside of the coordinating fabric. I tried to make the outside seam of the coordinating fabric at the mattress edge and the hang would be the center fabric.
With a 90 inch wide quilt top, I would use the 43 inch center piece, a 10 inch coordinating fabric on each side of center piece then then another piece of center fabric 18 inches on each side of the coordinating fabric. Remember your seam allowances, I am using finished widths. I know this equals 99 inches but when long arm quilting you need extra fabric on each side of the quilt. I then cut off the excess after quilting and use in another quilt top. I like to use my quilts reversable coordinating with my bedroom colors for the backs.