David has brought this to us :
Am I in a minority in being unable to set an accurate horizontal level
against a vertical surface like a wall using a spirit level?

There seems to be a degree(?) of variance between just touching and just
past the line for the bubble which seems to be beyond my ability to
discriminate.

Fall back is measuring up the wall from a flat surface.

A laser level might help (if I can get the level level, so to speak, which
comes back to the original problem).

What do the experts do?

The ultimate, which is always correct, is a water level. Just water
filling a clear section of pipe, but be aware that air bubbles can off
set it - always ensure any bubbles are out of the water. They work over
a short distance, or a very long distance.