How does cutting a pork shoulder affect cooking time?
Cooking by minutes per pound is far less accurate than using a thermometer.  The challenge with a pork shoulder is that it is made of of different muscles.  So, timing could certainly depend on which you are cooking...and, when using the charcoal grill, obviously the cooker temperature is going to be a factor.  So, get a good thermometer and rely on that.  Use the shorter end of the time estimate for an indication of when to check the temperature.