Butts are very forgiving , a temp. range of 50* or so won't hurt, it's the IMT that heralds the finished product. Judge time at 1.5hrs. / lb. and sit there drinking your brew and watch for , say 10* changes and move the intake just a little to regulate airflow and thus heat, keep the exhaust wide open and use only the intake, as stated , patience is your friend - be vigilant and your Butt will be Lip Smacking...