I dont have my antenna book near me but I thought the F2 layer was up around 250 miles. I believe the F1 and F2 layers combine at night to form a single F layer which is lower than that. It is not really critical as far as absolute distance away from the surface because as stated previously using trig, since the launch angle up to there is frequenty very low (20 degrees or so), the extra distance is not much at all. So for every 1000 terrestrial miles, it may be 1100 total skip miles if I am not mistaken. That is misleading because the signal has to go up 200 or so miles and back down 200 or so miles but that only adds about 100 miles to the total distance, not 400 miles. How about that? Trig is "cool".