For Straight of Grain
To figure the length of binding needed, take the length plus the width times 2 and add 12 inches.
Example: length is 68" + width 52" = 120" x 2 = 240" plus 12" = 252"
Divide that by 40" to get the number of strips needed = 6.3 round up to 7 strips. (252/40)
To figure the yardage multiply the number of strips by the width of your binding
Example: 7 strips x 21/2 " = 17.5 inches = 1/2 yard.