Forget adding numbers and 1/4 inches!!
I just make sure I buy enough fabric, and then, I cut generous strips of fabric in the proper width, which is the width I want plus 1/2 inch.
I straighten one end of the border pieces and then lay my border pieces one on top of each other down on the center of the quilt. I carefully line up one end with the edge of the quilt and leave the other end extending over the opposite edge, under which I place a small cutting board. I then use my rotary cutter and ruler to cut the border fabric even with the end of the middle of the quilt. I never even know the numerical value of that number. (I just CANNOT remember numbers!)
I have made some pretty great quilts this way, and if you are careful about laying down the quilt and the border pieces, it is VERY accurate.
I first saw this method on TV being used by a famous quilter whose name I have forgotten, and if it is good enough for her, it's good enough for me!
BethMI