I estimate my cooking time and start checking 30-45 minutes before that with my Thermapen. I check in several spots and average the results.
I'm not saying that probe thermometers are not a good tool in your toolkit, I've just simplified things a lot as I've progressed with barbecue. Speaking generally about all meats (turkey, brisket, pork shoulder, etc), the risk with a probe is you're only getting one measurement in one location. If that one location is measuring 160*F but the average measured in several spots is 170*F for whatever reason (e.g. the probe is improperly placed, not in the center of the thickest part of the meat, or in some meats, you've placed the probe in a fat pocket vs lean meat) you have overcooked the meat beyond your target internal temp and there's no recovering from that.