I would be very surprised if you hit 190 before 1 hour per pound on a butt. If that happened to me, I would move the meat temp probe to be sure the temp was accurate. Truth be told, I typically experience at least 1.5 hours per pound for a butt.
Instead of lowering the smoker temp, I would allow for at least one hour per pound, let the smoker go at 225 (I actually set closer to 235-240), and if the butt finishes sooner than expected, then take it off of the smoker, wrap in AL foil and put it in some towels in a cooler. This will keep the meat warm for several hours until you are ready to pull it.
Actually, in planning your pulled pork project, factor in at least one additional hour for the finished butt to be wrapped and held in a cooler just to let the juices redistribute through the pork before you pull it.
Good luck with the smoke! Let us know how you make out.