Reference level in RF receiving instruments

Hi all, 

I was wondering if someone could explain me or point me in the right
direction (some pdf, tutorial or similar) on how to choose Reference Level
for Spectrum Analyzer or VSA and how do I actually know that it is good or
bad. 

I am aware that I should probably maximize the signal by setting reference
level a bit above the signal and not too much but is there more to it than
that?

Also, I was wondering how FM receivers set their reference level (if they
have this option) or how  they deal with signals which are too weak?
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