Going along with what Noboundaries has already said, brisket is both ridiculously easy to cook, but at the same time, extremely difficult. Sounds contradictory, but that is the nature of brisket.
If your brisket is dry and tough, then it's undercooked. It's somewhat counter intuitive as with most other meats, "dry" always means overcooked.
The reason for this is that the moisture of a brisket doesn't come from the external fat or marbling, but rather, from the breaking down of the collagen/connective tissue between the muscle fibers. This breakdown starts at about 140 degrees but accelerates as you pass 160 and go higher.
Here's a photo of a piece of brisket that is undercooked: (click on the photo to enlarge it).
See the little squiggly lines between the muscle fibers that the black arrows point to ? That's the collagen/connective tissue still intact leading to a dry and tough brisket
Here's a slice of brisket that is just a tad bit overcooked: (click on the photo to enlarge it).
Here you can see gaps between the muscle fibers where the collagen/connective tissues used to be. They have broken down and rendered making the brisket moist and tender.
As to finished IT of brisket, it can vary based on a number of factors. Some might be done at 190, others at 195, yet others at 200, 205 or even 210. This is why you shouldn't cook brisket to a predetermined temp, but rather, cook it until it is done. It's "done" when you can stick a probe into the thickest part of the flat and it goes in and out with almost no resistance, like a knife through warm butter.