if you add all the sides together you come up with 270". however, if you add the vertical borders first, you will need 6.5" more on each horizontal border, which adds 13", making it 283".
you can get 5)pieces out of a yard if you are cutting WOF {40X6.5} so i would get 1.5/8 yds minimum. if you don't want your borders pieced, it will be different.
hope this helps!