I just answered in another post that the 3-2-1 isn't always perfect. A lot of times, the meat just falls off the bone. If that's the way you like it, by all means use that formula. Otherwise, adjust accordingly.
Lenny if spares I usually put them in and when I get 1/4-1/2" pullback I foil thats usually 2-2.5 hours then I spritz them well and into foil for about 1.5 hours then unfoil and back into the smoker till they stiffen a bit usually about 30-45 minutes
That last hour, or whatever time variation you choose, is to "set" the bark which has become quite soft from it braising time in the hot moist environment of the foil. It can also be adjusted to allow more cooking time if needed. I don't find I need more than 30 min. Bark usually isn't always something as desired on a butt, especially if it is going to be pulled.