There are several factors that can affect times. Outdoor temps, cooker temps, meat mass, how often the smoker door/lid is opened and how long, humidity, etc. No matter how good you are the times will fluctuate from one smoking to the next. I highly recommend a remote temperature probe when smoking. You insert the probe into the thickest part of the meat without touching a bone and leave it for the duration. It has a metal braided lead on it about a meter long that plugs into a base thermometer with liquid crystal readout. This will allow you to monitor the exact internal temperature of the meat so that you know when it’s done without over/under cooking it. It takes the guesswork out of smoking. After a little practice you can even predict when the meat will be done by watching the temperature rise.
I have a Taylor thermometer that I picked up from Target: http://www.target.com/gp/detail.html/sr ... B00004XSC5
. Polder also makes a good remote unit. You can spend less that $20 or as much as $80 or more for different ones. Some had wireless remote units that you can monitor from 100 feet away. My $14 remote thermometer works quite well and I wouldn’t want to go back to guessing.
Another option is the instant read thermometer. They work as well in reading the internal temperature, but with a few drawbacks. You have to open the smoker/grill, and you have to poke a hole in your meat which will allow juices to escape. This type of thermometer can be used on a hot grill whereas a remote should not.
Hope this helps.