You've probably also read that, "it will be done when it's done".
Due to the thickness of the meat, the amount of internal fat and the density of the muscle, see above. A normal done IT for a pork butt is between 195 and 210*. The best way is to check the bone when you get close to these temps. If it wiggles very friendly and can pull out easily, the meat is done. If it doesn't, give it another 5*. It will also have a very tender feel to the touch.
Good luck, Joe. :grilling_smilie: