The best thing I've learned about light usage in classes that included a "night shoot" is to practice several techniques.
I checked "Harries", because it's my first preference, but I'll use whatever suits the occasion. Whether I like them or not, I now practice a few techniques.
I found the Harries "fits" me better of the techniques I've tried, because it best points the light beam where my sights are pointed. Most others require some adjustment of my light hold.
But the Harries is more fatiguing if done properly for a long time, it is a hinderance if I would have to shoot around cover that's on my strong side (light would shine into the wall), and doesn't work well with Iso stance.
If I have to switch, I'll usually try to switch to a hold that I don't know if it has a name. I hold the light in the weak hand with the lens on the thumb side of my hand; then I'll press the tailcap switch back against my strong hand's knuckles (holding the gun) to activate the switch.
If that doesn't work, then it's the "cigar hold".
Then the "neck index".
Something interesting I saw in the last class I attended-
There were a couple of shooters who found that their chosen technique didn't work because they couldn't see. For some optical reason, if the beam came from the wrong direction, they "lost" the sights and/or the target. When they changed the hold, they were fine. One found he could only use a light if he held it on top of his head and let it shine down and forward.
Strange things. But a good reason to train- so we find these things ahead of time.