That would be math! Seriously, get some, not a lot, but enough to do your braids so that you can measure how much shorter the braided section is than the unbraided pieces. Then you will know how much more you need of each section so that the finished product is the length you want. Example – you start with four foot pieces, and when braided they equal two feet (I am making this up as far as the numbers go). In this case, if you want six foot finished pieces, you would need twelve feet of each. I hope that makes sense.
It is never the horse's fault