"more of a guideline" - yes and no.
It seems to me that certain
uncountables lend themselves to use with the indefinite article than
others. The ones that do almost invariably take the indefinite
article when the appropriate conditions apply.
So
knowledge and
sadness go in the one group and
grit goes in the other group.
At least that's one theory. Another possibility goes as follows:
The use of
a/an can be paraphrased as
a kind of.
a good knowledge of is then
a good (thorough) kind of knowledge of.
a kind of doesn't make as much sense with the adjective
true:
a true kind of grit? as opposed
to a false kind of grit? Perhaps the only kind of grit is true grit, so the use of
a doesn't make sense.
CJ