Here's my method, and also got a question.
No matter the size of your bone in shoulder, put in on, fat side up at 250-275ish "grate" temp and hold that temp. Don't open that lid until about 4 hours, just maintain a steady temp. Consistent temps will help your cooking times.
When you check on the butt and notice the fat cap separating or splitting from itself, take it off. You will also check for your "bark" and color. Double wrap in foil, adding a little liquid and back on the pit.
Give it a few more hours... three or four, pull it and run a skewer or a probe through. Obviously if it's tough, needs more time. Wiggle the bone also, if it doesn't move, needs more time. Go by feel, not temp.
Once your happy and have pulled it, rest it for an hour or so either wrapped in a towel in a cooler or just on the counter still wrapped.
Pull it saving the juices, sprinkle more dry rub over the meat and pour in juices.
Don't worry about IT's (internal temp) just go by feel.
This is just my way. Works for me so I stick with it. Everyone does it the way it works for them. You will too.
Question-Do you buy your shoulders from the same place? If so, try another shop as well. Like a quality butcher shop. All pork is different grades just like beef. Might just be what your starting with. I buy shoulders from my grocery store when on sale of course. Good, but not the best quality. If I'm cooking for a party, I buy better stuff from my butcher.
This is a good read and sorry for he long windness.
http://www.meatscience.org/TheMeatWeEat/topics/fresh-meat/grades-of-meatSent from my iPhone using Tapatalk