IMO, its a waste of time and charcoal to cook one butt. Fill up your pit! It takes the same amount of charcoal to cook four butts as it does to cook one. Throw on a few wings or moink balls for appetizers. Let everyone eat until they are stuffed and then send them home with a plate. Freeze the leftovers in ziploc bags and reheat in a pot of water.
If I'm just sitting around shooting the breeze, I mop with a vinegar based sauce every half hour or so. If I'm cleaning up the yard and patio, etc, I don't worry about it.
I try to keep the pit at 250. Foil after 5 or 6 hours and continue until the probe slides in or the bone slides out. After you foil, you can crank up the heat a bit without drying it out. Mine usually take 8 - 9 hours. Let it rest in the oven or a cooler for an hour or so before you pull it.
Just my 2 cents.
Good Luck! Have Fun! Let us know how it turns out.