I put the probe in the thickest part of the lean meat. Now, keep in mind, the probe is just an indicator of when to start checking the brisket for tenderness. It won't necessarily tell you when the brisket is done.
I put the monitoring probe in the thickest part of the lean meat (usually the point) and when the probe reaches my desired temp, I start probing with my Thermapen all around the brisket until the probe goes in the meat like it were being stuck in butter. Tenderness is the indicator of doneness more than temp.
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