When it goes from brown to black. Remember, the main thing you're going to accomplish is to add flavor by caramelixzing the beef (Maillard process). The meat is so thick that you won't hurt it if you overdo it a bit, and by the same token, you don't lose that much if you underdo it a bit. It'll also jump start the cooking process a bit by raising the meat temp much faster than if you just stuck it in a 250* smoker.