My method. Most fabrics have usable about 40 inches wide. Most times it doesn't matter whether you do WOF or LOF.
So divide 80/40 = 2 widths to make your length.
2 widths x 56 inches = 112 inches total length of fabric
112 inches divided by 36 inches in a yard = 3 yds (108 inches) + 4 inches = 3.11 Yds.
Round the .11 up 1/8 (4.25") or 1/4 (9 inches)
Edited to add: When I make my backings like this, I use a 1/2 inch seam allowance and iron the seam open. This takes most of the bulk out of the seam.
Last edited by Barb in Louisiana; 08-21-2017 at 08:21 AM.