Sorry about not seeing this earlier, but I've been moving my daughter to her first house.
In reality, we should be talking time per inch thickness of meat, but so be it. Time per pound is close enough. On a big rig, you don't usually have to account for any additional time for a big load unless you put them in icy cold. I usually find out what my largest piece of meat is, calculate the cook for that piece. Then add 2 or 3 hours for FTC (I just throw them into a cooler I have dedicated to the task) to be safe.
If my biggest butt is say 8 pounds. I'd plan on a smoke of about 13 to 15 hours. 8 * 1.5 hours per pound = 12 hours and then add 1 to 3 hours to make 14 to 15. The 1 to 3 hours is safety if they actually take 2 hour per lb, which is rare in a big rig. If they start finishing early, just chuck them into the cooler, they'll stay warm for several hours. You could add 4 to 5 hours if you feel you need that much safety. That 1 to 3 hours also allows me to pull, slice and get everything else on the table. When cooking several pieces of meat, I periodically take the temp of each piece - especially near the end - and take out each piece as it gets done. The timing doesn't include getting the rig up to temp. I usually plan on preheating an hour or two on my rigs - some rigs take longer to stabilize.
Often if your cooking temp is 20 degrees off for long periods of time or the entire time, then the you add 10 minutes per pound for higher, or subtract 10 mins per pound if lower for each 20 minute. Each time you open the lid is also more time, but not as bad with a big rig as say a Bradley. I usually don't open it up until several hours into the smoke - to check temps and to mop.
If you use rub - go liberal with the application. You are seasoning pretty good sized chunks of meat.
Very important is that this is based on 225 deg cooking temp at the meat level. If you go much below that for an extended period time - like if you fall asleep between 1:00 and 4:00 am (of course, I've never done that - now ducking the lightning) - the time will extend. If I do an overnight - with the rigs I have left, I'll usuallly set my alarm for every hour on the hour. Fall asleep then wake up each hour to tend fire, mop, take temps, etc......then set alarm again for one hour and fall asleep again. Once you get used to it, it is surprising how much sleep you can get. But you have to remember to set the alarm for every hour. and have a loud alarm.
Let me emphasize again the cooking temp is at the meat level and does include the radiant heat from the fire - if not an indirect smoker.
