You really want to cook butts in two phases. Start them off at 225-250 and leave them there for 14-16 hours. You will see the temperature of the butts move up gradually for the first 10-12 hours and then stop moving once they hit somewhere around 175-180. (This known as the plateau and drives first-time butt smokers absolutely nuts.) At that point, the collagen in the butt will start melting down. This will take around 4 hours. Once the collagen has broken down, the internal temperature of the butt will start moving up again. At that point, crank up the dome temperature to ~325 and shoot for an internal butt temperature of ~200-205.