Everyone makes good suggestions, there is a 100 different ways to do it and each is good. me I do 220 degrees and pull at 203 its like my magic goodness number. I figure 2 or 2 1/2 hours per pound cause I don't use the Texas crutch. I usually buy the biggest butt I can, normally a crypopack with both pieces. If I put it on today at 220, I figure it will be ready tomorrow about the same time, give or take a couple a hours.
All ways work, my suggestion/tip is to cook it a day ahead! That way no matter what the prk does to you its a no strain. I prefer it that way because you can use a little finishing sauce after pulling, and allow it to sit over night and its soooooo much better. Seriously.
The thing about butts is the pigs never learned to tell time. My last butt, 9/10 pound butt was done in 13 hours! I don't know how or why it happened, but it did. What I am saying is butts are like women, everytime you think you have 'em all figured out them mess with your head!
Seriously, I highly recommend smoking the day before cooking, adding sauce and resting over night. serve the next day in a crock pot add a touch more finishing sauce if needed before serving. Its the only way to do a butt! However you decide to do it.