make a one inch section of the weave and mark the ends of your strands right at the end of the weave. Undo your work and measure each strand. That will tell you how much cord you use per inch of finished weave. From there it's simple math.
Another way to do it, is eyeball it on the person or application you are making it for. It is not as accurate as doing the math, but it will get you really close. Normally when I eyeball it, all I need to do is trim a bit and burn the ends to get it just right. Also I recommend a Jig. They come in really handy.