Hi Joost. Im big fan of your plugin:) so I want to give you one friendly suggestion.
I know that there isnt any formula of the best keyword densitybut I think that your plugin encourages the keyword stuffing. Especially to the non experienced bloggers who can be lead by your plugin anal
Many sites and blogs got penalized by google these days (including my friend's blog) because of keyword stuffing (I don't say thy are using your plugin and it encouraged this cases) but Im sure that your keyword density (top border)as f.e. "The keyword density is 5.48%, which is great, the keyword was found 16 times" on article of exactly 300 words. is far too much for 300 words article to have that word 16 times included. Maybe the lower border from 2% or 3% is ok but 5,5% in 95% cases will be keyword stuffing.
You should either lower this border or maybe to put warning (after 3%) to not overuse or miss use this numbers only because in Page Analysis say its ok.