Well Dont let her sink to low. Then the fire would be out. I would open 1 vent 15%%. Wait 30 min and check temps. I Bet your Butt will turn out great if you just pull it when its tender enough. And it could be cooked @ 300 and still be good.(if your rub isent heavy on sugar) But if you have all lower closed,and wait for the temp to be in the low 200 and think you can open up a vent... It can be tricky...And the fire could be out. I know that my WSM(22,5") dident put itself out with the lower vents shut(first cooks).
And if you use water add ice...Or Really cold water to get the temps down.
Keep an eye on her. Dont wanna make it harder then it needs to be.