If you are wanting to go for 225 then I'd use water to help keep temps down. Fully loaded ring of charcoal and just a 5 or 6 lit coals. The slower you bring it to temp the easier it is to lock in on the WSM in my experience.
I'd expect a typical 8 lber to take a bit longer than 12 hours at 225 unless you are foiling.
Me I'd go without water in the pan and let that temp ride up to 300 plus when cooking a butt. no foil and still just as good of a result as low and slow.