"I'm reading that the consensus is don't change gain once set? This is contrary to what I've been told verbally by other drivers."
Setting once might work if every outing was exactly
the same....not so much in the real world.....
Better to set once for every outing.
My routine, start TV, after idle stabilizes, put in gear adjust gain to stop & hold, keep it at that threshold, ck again at 10-15mph to dial in. Setting to just before trailer lock-up has not worked well with our Classic & TruControl combination.
Major load change may require further adjustment.
In my experience, (Tekonsha & TruControl), more gain does not
effect engagement timing. I have found that if your controller is wired into the brake light wiring, your trailer brakes will engage before the master cylinder applies pressure.