I calculate the fabric for my borders and binding like the. seem much simpler to me.
add all four side of your quilt together and divide by 40. this is the number of strips you will need.
multiply the number of strips by your desired width of your border.
example: 50X60 quilt
50
50
60
60 equals 220 divided by 40 equals 5.5. round up to 6
6 times a 3" border equals 18 inches of fabric