It depends. It depends on the reading grade score you use. If we take
the Flesch-Kincaid reading grade level then this gives us the level of
schooling that a reader would require if they were to read your text.
Ie: if you get a reading grade level of 7 then this means that the
reader would need to have an education level equivalent to year 7
(based on the US schooling system). A score of 13 or more would
require a university education level etc (it get's a little inaccurate
at high numbers). Again this is all just averages and theory,
obviously one year 7 student will be different to another.
As for the best number, again that depends. For a government website
you would want to write content that can be read by the majority of
the population. In America the average reading grade level is about
year 9. This means that the average person on the street can read at a
9th grade level. So you should be aiming for your content to be within
the 7-9 reading grade on the flesch - kincaid score (below 7, and the
text can become too simplistic and it will be hard to get your meaning
out).
You should take all of this with a grain of salt, as the score can be
easily manipulated. The Flesh-Kincaid score really only cares about
sentence length. The shorter the sentences the lower the grade. But
you could imagine a situation where you have very complicated words in
short sentences. It's still probably going to be difficult to read!
-Joel