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@ OP-BGE has a temperature off-set for indirect cooks that will really affect your cook-time. Basically the dome thermo reads 20-40*F hotter than the cooking grid. Longer the dome is shut the narrower the gap but as Tweev_tip and others have mentioned, go hotter and let the BGE do the work. I still find around 2hrs/# with the dome 270*F (+/-) for pork butt cooks. Plan long and if finished early you can always pack and wrap.
BTW-since you have cooked butts before, then you can scale what you have seen in the past to this adventure.
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