I have run 21 hours at 235 degrees with a full ring of coals on the 22"WSM using the minion method. For an extended smoke I light 15-20 coals and set them on top of a full ring of Kingsford competition with cherry or wood chunks burried in. I normally keep the top vent wide open and adjust the bottom to get to temps.
I typically smoke a little hotter now with pork butts at 250-275 and ribs at 275-300. I really do not find any difference cooking pork butts at 225 as compared to 250-275 except they don't take as long. Also, don't be afraid of a little temperature fluctuation, it won't hurt anything.