To save fabric, you typically want to add the long borders first, and then the short borders. This minimizes the yardage you have to purchase to keep each border one piece.
As someone else mentioned, measure through the middle of the quilt to determine how long the longest border should be.
If your long measurement through the middle is 80 inches, you will need to cut your long border strips 80 inches long. As long as the *finished* border *width* is less than 10 inches, that is how much yardage you need to purchase (although I always add about 1/4-yard for wiggle room.) 80 inches + 10 inches for wiggle room = 90 inches of yardage to purchase, or 2.5 yards.
The border *width* is important because you want to fit all 4 widths into 42-inch-wide fabric. If you want a 12-inch border around your quilt and do not want to piece the borders, you have to purchase 5 yards of fabric because 4 times the 12 inches of border width = 48 inches (too wide to fit 42-inch-wide fabric).
Once you have purchased the fabric, cut the two long borders first (to the exact length you measured through the middle of the quilt) and sew them on to your quilt. Press and measure through the middle of the quilt in the other direction to determine how long the short borders need to be. You will be able to cut those short lengths from the remaining fabric.
With this method you do not need to add any seam allowances to the length of any cut. The middle measurements are exact measurements for length. The only seam allowances you need to add are to the *width* of the cut. If you want 9-inch-wide *finished* borders, for example, you need to the cut the strips 9-1/2 inches wide (or, as I do for wiggle room, 10 inches wide as I can always cut off the extra when I start the binding step.)
That is the simplest method I have figured out to determine yardage for a border.