I pull the chuck roast based on time. I try for roasts that are 3-4 pounds. I cook them for 6 hours and then foil them for 3. If the roasts are closer to 4 pounds, I go closer to 6 1/2 hours unfoiled and 3 to 4 hours foiled. It's basically 3 hours per pound, but you don't have to go a full 12 hours if the roast is 4 pounds. 10 to 11 hours will work.
They're kind of like butts, I don't think you can screw them up to easily.