I agree with your principle, so my rule of thumb is, if the top of the beam doesn't hit the road, it's too high. So somewhere in the spectrum of pointed straight down, and top ray of the beam being horizontal (touching the horizon), is where you want the light. There's no perfect place in that range, but like others have said, the point is to see where you'll be riding. The faster you ride in the dark, the further forward you need to see to avoid obstacles, thus the higher angle and more beam spread you'll need (thus the more powerful light you need to fill the more spread beam area with equivalent brightness)