78 1/2 inches is 2 yards 6 inches. So you have enough fabric to cut the border lengthwise. When I do this, I lay out the length of fabric on my floor. I then measure the width needed every foot or so and make a chalk mark. Then I use a yardstick to make a continuous chalk line to the required length. Then I cut it. Of couse, all of this is while I'm sitting on the floor! Be careful not to include the selvage in your width measurements.