Maximizing tail light visibility

Just a few days away from the shortest day and still a few months till it gets less foggy than this.
So visibility of cyclists by motorists is down - and will get worse through smeared windscreens if
the rains ever come.

I have seen many cyclists with excellent rear lights (evidence they are thinking seriously about
visibility and safety), but who clip these relatively expensive and efficient lights to their
backpacks and the bright beam shoots up into the sky, not back into the eyes of motorists.

Recommendation: get a friend or spouse to drive behind you at night and report on your visibility.
No good holding the tail light in your hand 10m away to be admired - gotta be a full simulation:
usual placement of light on the bike or pack, regular commuting route, same time of day, friend
driving travelling at traffic speed etc.

Keith