The crux is to put the fan
where it will best force the damp air in the bathroom to circulate, mix and
draw fresh, drier air in, from say the corridor or bedroom adjacent, while ejecting humid air to outside. The speed that gas and water vapor molecules move is incredible, [hundreds of mph], so the air in a room will become homogenous eventually. Where you put the humidistat is thus pedantic, IMHO, considering the use-ratio, but perhaps it's easier to wire in the ceiling? Cold spots on walls can create dewpoint conditions creating condensation that mold can live on, and these spots are generally low down or in corners out of drafts. But surely the main job for a fan is to get rid of nasty niffs
from the khazi?
A delay-off timer on the light circuit works great.
I have one bathroom fan with a humidistat built-in - works just fine but 'her indoors' can't leave the danged thing alone and keeps fiddling with the adjuster knob, either because it didn't
go on when she ran a bath or it was already on
when she entered the room. Keeps her busy I suppose!
[wiping condensation off the mirror for a shave, my pet hate! Why can't we have heated mirrors? ]