I believe that the higher the back the faster it stabilizes but it will also bring some noise ans speed reduction (not that noticeable). I would keep it around 3/4 or lower and the length can vary between 4" and 5 1/2". It all amounts to personal preference.
If you start out burning high and long( one arrow so you dont mess all the finish up on the rest)shoot them and see if it fly's the way you want, you can always burn them lower and shorter.Just keep the ribbon clean(a small soft metal bristle brush works great), and let the ribbon get hot again before you rotate the next feather on it (makes a cleaner cut)and take your time. Good luck JMHO