I have got to agree with Danny, I got the Maverick ET-732 for a present a couple of years ago, and it just takes all the hassle out of the job. You know what your smoker is doing and where your food is at. You can set alarms on it, so if the temp goes too high or too low, then you know before it's a problem.
OK, it costs a few bob, mine had to be prezzies in lieu, but it takes the guesswork out of things.
The bolt on thermometer I fitted to my smoker is not anywhere near accurate, and it differs across the temp gradient.
I would say you need to know what both the smoker temp, and the core temp of your food is to get it right, as the variables such as the ambient air temp have such a big effect. I did the Xmas joint of Beef in the smoker, and couldn't get it to 220f, I had to put old blankets on to insulate it. Then it was happy. I hope that helps.