I was going to say the same thing. You need a small border between the quilt top and the 1/2 square triangle border. Decide what size you want the 1/2 square triangle to be. Divide the width and length by this number. If you get an odd number, round up to the next whole number to determine the number of squares you need. Cut the small adjustment border so the quilt top will be equal to the border strips. I know that's as clear as mud, but it is the best I can describe.
Hope that helps