For a quilt top of this size, I would simply measure through the middle and use that measurement to cut the first two borders. After sewing them on, I would measure through the middle again and use that measurement to cut the remaining two borders.
Once the first border is put on, I would follow the same procedure to add the second border.
What happens then is that you are stretching/easing sides to fit the borders, so that opposite sides end up being equal.