Amount Of Charcoal Used - 22 vs 18 WSM

Chris' instructions recommend 2 chimney's worth of charcoal for smoking a turkey (at 325º), but that's for an 18-inch WSM; will the same amount work for my 22, or should I increase it? If so, by how much? The bird's an 18-pounder, by the way, so it should be a 4.25 to 4.50-hour process....

Two chimneys will be plenty, as long as you're not cooking out in cold and/or wind. Keep an eye on the fire and if you find the coals are burning down fast, throw a shovel-ful of unlit on top of them every 30 minutes or so to keep the fire burning at max heat. After I add coal I like to give each leg a few whacks with the back of the shovel, up where it meets the bowl. I think it helps the ash settle out and the new coals to settle in amongst the existing ones. Then again, maybe it's just a silly ritual I've developed.