I put it in the thickest part of the flat. Keep in mind that there is no perfect temperature for brisket. While in my experience it averages around 200, I have had briskets done at much lower and slightly higher temperatures. When the probe slides in like butter, it's done.
Mister Bob, Pitmaster - Smokestack Lightning, Bad Ass Barbecue. KCBS CBJ #31759 - IMBAS Certified MOINK Baller