Cook a few or several eight pounders at a time and see if they all get done at the same time. I wish. Sometime a heavier butt won't be quite as thick as one that weighs a pound or more less, and it's anyone's guess which one will get done first. Timothy is spot on about foil helping with cook timing, and that's a big reason why BBQ competitors most all use it. I do as well, especially if I can see that a butt isn't gonna be done on time. Dwaine's also right about giving enough time to a cook, although I don't cook that slow. If not foiling, almost all of mine get done in 1.5-1.75 hr per pound cooking at around 250*, grate temp.
Only thing I can really add is that remote digital temp probes really do help in timing cooks. I'd always have a butt probed in my 18.5" wsm since I got the grommets almost from day one, but I went back to cooking old school blind with no probe in any on my big 22.5" and found it to be a little more difficult. Finally, I cut a slit in the top of the middle section a couple weeks ago so that not only I can monitor a butt, but I can check grate temp, as well. It might be way off from what temp the heat is that hits the meat when they're still cold, but it's more accurate than going by lid temp. Sun and wind are big factors in affecting lid temp, not to mention that you have to factor that the vent temp is somewhat higher and the gauge reads somewhat lower than actual cooking temps. When I changed to measuring grate temp in my 18.5", my cooks automatically shortened and no more 2 hr/lb butt cooks. The slit in the middle section makes it a lot more convenient than using the grommets, though.