If you are using cotton quilting fabric for the backing the usable width is only 40-42 inches. If you were to seam this with the seam running vertical on your quilt (74 1/2 inch ) you need your quilt length plus 4 inches (backing should be about 4 inches bigger) times 2 to get the length of cotton quilting fabric needed for the backing - approximately 4 2/3 yard.
If you were having your backing seam run horizontal (61 1/2 ") you would need approximately 4 yards - ((61 1/2 " + 4) times 2).
If you are using wide backing you only need your length (or width) + 8 inches (4 at each end).