One other thing to note is that it's perfectly acceptable to have your butt finish hours and hours before the planned dinner time. Actually, it's desirable for this to happen. When the butt passes the bone pull or the probe test, take it off the grill, wrap it in foil and let it sit for a couple of hours before pulling.
If it's done WAY early, do the same thing, but put it in towel lined cooler for the rest period. Butts will hold for over 6 hours this way.