One thing you left out that I think helps a lot as far as locating goes: the rats will always face the rat king! If they face left, he's to the left, if the face right, he's to the right, and if they're standing, he's in that column!
303! And I had two hearts left to spare thanks to careful chest-finding. Absolutely incredible game, I'm so glad I didn't see any spoilers. Like someone else said, the monsternomicon was indeed all I needed (and a bit of luck, I admit).
