Ellen (?)Thomas, one of the early links posted; if we've got 10M species at present, and we take the "estimates" in the links that there have been 100 -1000 extinctions for everything we see today, that gives us rough numbers for the first approach.
Otherwise, looks like you've got the gist of things --- mostly an attempt to "fence" some of the numbers in a little --- none of this qualifies as limits or boundary conditions on a problem statement.
I've been going to play the same game with the categories listed in the "earthtrends" link evo posted --- not far enough up "the list" at the moment (or, I've not worked far enough down). Doubt there's data enough to get past the KT event.