Have done several butts on a kettle grill and I find that my grate temp stays right around 250 if my dome temp is 300 or so. The butts take anywhere from 7-9 hours and I agree with disregarding the pounds per min thing and just be mindful of your internal temp (200 for me)
Also, I once had a pitmaster tell me once the only test he used to test for doneness in pork butts, is to grab the shoulder bone and if it slides easily its done, anyone use that tech?
Assassin gravity feed smoker
Custom Pitmaker Magnum Sniper Smoker
Weber performance Kettle