If you put a sign at eyeline level, only the people at the front can see it. Everyone not at the front will see the back of the front people's heads, blocking any view of the sign.
Perhaps I can partially repay the favour; this post (and the things it links to) talks about photographing memorials in order to read them. It's mainly about light.
I was with Eon (for ages) then moved to Bulb (which earned both me and Richard Herring £50 cash) and when Bulb exploded I moved (by default) to Octopus.
Bulb/Octopus have been fine (they recently upgraded my old Electric Smart Meter, and installed my first Gas Smart Meter)
