Don't be too quick to discount Smoke and Spice's temp/time guidelines. Those guidelines are for smokers in general, and they're spot on where they describe charcoal-fired water smokers and their bell curve on pg. 11 and then this: "...You have to rely on the average heat level, which can be difficult to determine accurately, to estimate the cooking time required for a dish...With a little experience, the cooking process becomes easy, but many people find it frustrating at first." But that probably wasn't so much the issue here since 4.16 lbs means it was only half a butt, and that means that all bets are off. That's because popular BBQ hr/lb guidelines only really work well with avg. (whole) cuts of meat.
Still, as smokers go, the wsm cooks on the slow side. However, it seems to cook more efficiently if you get the temp up to where the pan simmers and steams. I've cooked at avg. temps of 225 overnight and more like 240 by day, and it's amazing how much difference 15* can make in the length of a cook on the wsm. Maybe that's why Weber suggests that 250* is the perfect smoking temp target. Hit that relatively early and the wsm isn't so slow, but don't be discouraged or too disappointed, though. It's kind of fun to learn how to estimate cook times, and not as hard as it seems at first. It just takes practice and experience, and taking good notes helps as well for future cooks.
Give yourself plenty of time, and if you're BBQ is done too early, tent it a bit to let the steam off, and then wrap up tight in foil to hold in a warm environment for a couple or even a few hours. I call the faux cambro (preheated cooler) the wsm-users crutch, but most of us do it from time to time, and it won't hurt a thing as long as you don't hold to long, too hot, thus overcooking the meat with the "carry-over" heat.
Regards,
Dave
