Meaningful Sequence/Letter spacing

Is there a requirement that the letter spacing must be spaced at a minium length?
 
There is a statement at Visual Presenation that line spacing is at least space-and-half within paragraphs, and paragraph spacing is at least 1.5 times larger than the line spacing.
 
Is there a similar minimum length requirement for Meaningful Sequence. I am not trying to adopt Visual Presenation. It is only used as an example.