The 1.5 hours per pound is only a guideline; a given butt can take a considerably longer or shorter time. You're shooting for a chamber temp of about 225o until the meat hits an IT of 200o. Then FTC it for an hour or so before pulling.
All in all, an 11-pound butt can take from under 10 to about 20 hours to cook (plus FTC time) - plan your schedule accordingly. You can always do it ahead and reheat for serving.
