Interesting.......
I set QNH before taking off, as it's easiest to do it then, and I can confirm it's right as I know the field elevation above mean sea level.
QNH = Newlyn Harbour = Sea level !
On returning to an airfield, you know those pre landing checks, where you say "instruments", that reminds me to set QFE for that airfield.
All very simple really.
EXCEPT
came unstuck the other day, "cleared to transit the overhead not below height 2500' "
and I was on QNH and I selected 2800, and the airfield was >300' amsl. Oops.