What I would do...
First off, choose a temperature you want to cook, be it 225, 275 or 300, and use a proven method at that temperature to run your cook. Believe it or not, the cook temperature affects the final temperature.
Yes, run a temperature probe in the middle of the flat, so you can learn when the flat is approaching the proper temperature for your chosen method of cooking. Once it reaches the temperature for starting to probe, remove the thermometer probe. From here, it is completely based upon feel. Probe every 30 minutes to 45 minutes, using an ice pick or metal skewer, and what you want is a very tender resistance, like a butter knife through room temperature butter. Remove and let rest, for at least 30 minutes or until brisket is at 165°F or so.
If you cook at:
225°F, start probing at 190°F
275°F, start probing at 200°F
300°F, start probing at 205°F