Just from my point of view as I have done a lot of small flat cuts (as sometimes that is all that is available) try using some liquid to finish it up. What I normally do, is smoke it with a set temp of 250 degrees, till the bark has formed pretty good, and you have good color (around 165 internal temp). I then either take a small aluminum throw away pan and add some beef broth with a little worcestershire or whatever you desire, and fill it about a 1/4 way up the brisket. Its hard to make a tender flat, especially a small one on the thin side, so I essentially start to braise it after a while. You can either leave it uncovered, but I try to foil it up pretty tightly with a foil cap. Start poking a toothpick around the 195 IT temp, once it slides through like butter, your good.