If I am out in the dark in a dark area with no street lights or other vehicles, I try to set my light so that I have roughly 75 to 100 feet of well lit ground. I am guessing that is how much distance I would need to stop if I was going downhill at 30 mph, that includes the time delay to decide if I need to stop if I see something.

But I usually have street lights and other traffic when I am out in dark conditions, then I usually try to aim the light so that I can just barely see reflections from parked car taillight reflectors.

My lights, I keep the bolt tight enough that I can adjust the light angle with my hand if I need to adjust it, but not so loose that a rough road or severe bump will change the light aim.

I have never used an Edelux, but if the switch has 0, 1, and S positions, I am guessing (only a guess) that 0 is off, 1 is on, and S is on in dark conditions based on a light sensor. If there is a sensor, there might be a time delay so that your light does not think it is daytime when you have a car headlamp coming towards you. A google search should help you figure this out.

I think fork crown is most common, as that is out of the way for handlebar baskets, bags, etc. And is less likely to be bumped there. But I see nothing wrong with handlebar height if that works for you. Too low, such as axle height will produce large shadows behind any road debris such as leaves, or large bumps. I have a light mounted down there on my errand bike, but that bike is only ridden near home on streets with street lights, I use it more as a to-be-seen light, not so much a light to see with. But from that experience, I can say that having a light up higher is better to reduce shadows. If the light is really low, a big pot hole looks the same as a shadow behind a small bump.