If it's possible to say anything
uncontroversial about Johnson's prescriptivism, it would be that
Johnson is more prescriptive than most modern lexicographers, but
also less prescriptive than most of his contemporaries expected
him to be. I'd remind you, though, that a dictionary needn't be
prescriptive in its intentions to provide a definitive standard;
it's possible to be authoritative without being authoritarian.
The OED, perhaps the most thoroughly descriptive
major dictionary ever completed, is today routinely used
prescriptively. A search of LexisNexis or similar databases for
the phrase "according to the Oxford English
Dictionary" is instructive: linguistic scolds routinely
turn to it for authoritative guidance on what's right and wrong,
even though its editors from Murray through Simpson explicitly
disavowed any such intention.