I never seen a 20 lb butt, but, when i get em, it's usually 2 butts in a package ... I would smoke it like 6 hrs, then double wrap with foil, put meat on shiny side of foil, then put your thermometer probes in em, bring grill temp up to like 275, and get an internal temp like 203 deg. then, break out the coleslaw ... lol, southern bbq , you always put the pulled pork on a bun, add salt and pepper, add a little bbq sauce, then a big scoop of coleslaw right on top ....