MIK, Lobo & Fungus get points for artistic creativity, His Royal Trout gets point for grammar.

Pyroxian gets points for mathematical correctness.

It's 5/5 * 1/5 = 1/5 = 20%

Designer Notes:

Professor Grabblefax is insane, yes. This neatly explains his random, haphazard selection of mutagens, his omission of the dreaded Omega Mutagen (and Pi, which he turned down), and his rather odd single-labeling system which (as rightly pointed out) is simply a bizarre, wrong-headed way to label a mutagen collection.