The card is set to 42 inch goods. Change the fabric width to 108 and watch the yardage drop. That drop is why wide backing exists.
Open Backing in the studio. The form on this page is that calculator.
If the back is only a little too wide for a bolt you already love, the diagonal backing calculator will say whether John Flynn's seam saves anything. At queen size it usually does not: 98 inches is well past one and a half widths of 42 inch cloth. Buy the three panels, or buy wide backing.
More bed sizes are in the quilt sizes guide.