FJ-
I would think that it would be hard to get much more consistency than the 2 hours per pound rule of thumb. Given the variables in each cook (weight of meat, shape of meat, proportions of fat and connective tissue in meat, type of fuel, temperature, water in the pan, sand in the pan, top grate, bottom grate, sunny, cloudy, rainy, windy, etc, etc.), well, you get the idea.
Having said that, when I do butts (Mr. Brown or otherwise, doesn't matter), I usually have 6-7.5 lb, boneless pieces from CostCo, I target to cook at about 230 degrees, usually on the top grate, and they generally take 12-16 hours.
Hope that helps some.
R