The distance from the top of the Esbit cube to the bottom of the cook mug varies as the cube burns. I would have to go measure some of these that I've built over the years. As I recall, it needs to be at least 1.0 inch, but probably no more than 1.5 inch. You also want to leave some wiggle room in there in case you want to stand the cube up on end instead of down flat. Compare my numbers to what you find on commerical Esbit burners. If you make that dimension too small, then the wrong part of the Esbit flame is hitting the cook mug. That results in extra soot deposited and reduced fuel efficiency.
Elevation off the ground is also a bit flexible. Mine all leave the cube at least 0.5 inch high. I believe I built one that left the cube more than 1.5 inch high, and it didn't accomplish anything except to use up more titanium metal in the burner construction.
You must have a windscreen that will block all wind blowing in from almost any angle. However, you need to have air holes or vents or an opening around the ground level so that cold air gets sucked through there, up slightly into the burning flame, and then the hot air goes straight up from there. Also, if your windscreen is so tight that you can't get your finger between it and the cook mug, then you are probably cutting off the exit air flow, which is not good. As a general rule, I leave anywhere from 0.5 inch to 0.75 inch of exit air flow space around the cook mug. On a mild summer day, I will use only ordinary aluminum foil for a windscreen. As the weather conditions get more challenging, it is nice to have something a little more foolproof.
There is always a fool who can overcome any foolproof scheme.