Sorry for bumping an old thread, but I've made a neat little discovery. For those of you who have slow fingers (like me) and are trying to do FH -> N-air but are finding it extremely difficult to get the timing down, I have a trick.
To do the move using the "normal" method, you have to make sure you get a full button press on your jump button, and then quickly hit A to make sure your N-air is inputted immediately after the jump. The frustrating part is that if you do the whole process too quickly, you often end up not pressing the jump button long enough and short hopping instead of full hopping. If you do it too slowly, Snake full hops but then your N-air doesn't come out quick enough for all the hits to come out.
Well I discovered a trick the other day to make this move very easy: You do a full button press on your jump button, and then keeping it held down, you quickly roll the bottom your thumb down onto the A button. Essentially, when you've finished inputting the move, you will have both buttons pressed down. If you've done it correctly, the whole thing should be one fluid motion. It's a great way to guarantee that your FH -> N-airs come out correctly every time.