I go by feel mostly, using temp. as a guide for when to start looking for that "feel".
Take my last BB smoke for instance...due to colder nighttime temps (25ish) and a slight breeze which was not there when I went to bed, I extended my cook time significantly. 20 hours later, I was only at 182, but, I decided to pull them anyway. Let them rest for 45 minutes and as soon as I started pulling them I knew I had made the right call...pork was incredible moist and pulled easily!
That said, you have to do a few cooks to get a feel for the meat, the smoker, and your environment. Remember, you are dealing with fire and air, both of which vary according to where you live and the climate therein.
Get your smoke one and do not be afraid to think "outside the box".