Seriously, quit worrying about the temperature. Its only good for two purposes, insure safety while learning and to give you an approximation while learning. It will never be the exact same for two pieces of meat.
In the begining smoke all you meats at the same temp to get a good feel for the time/ thickness/ density relationship. Once you have it figured out you won't need that thermometer. Pinch it, poke it, stab it, its OK to play with your meat. People were making far superior smokes long before there was indoor plumbing or electricity.
Use that temp. probe as a learning tool, but don't depend upon it as a crutch. There are too many variables on large pieces of meat to be able to do that.
You have the nice and slow part down, now its just waiting it out.
Oh BTW, I took a tip from a fellow down Austin way. I don't trim, he does a little but not for the reasons others do. He trims both fat AND meat off to try and establish approximately the same thickness and width. He doesn't do it exactly but it makes a huge difference in being a consistent cook time. You always get that thin piece of flat on the end opposite the point end, you know you can't do it proper justice so cut it back some use that meat for chili or other things. Don't just try and hack out all the fat, just try to smooth it all around so you get a good air flow around the brisket. You are an artist, a sculpture. And use your fingers, squeeze it, poke it, pick up a screw driver and pearce it! Do it before you put it ion the grill so you know what raw feels like. Toothpicks for me have too much of a point on them and just don't give me a good feel for the meat. Get a new unsharpened pencil. I use a large screwdriver, you can always find one in the back of someones truck to test with. Just don't let the women folk see ya.
Biggest thing is don't worry about it, when I stopped worrying is when I started wondering why I was having so much trouble, there is really nothing to it. Have a cold beer, tell a few lies, work on those laugh wrinkles around your eyes and don't get smoke in them. Relax and have fun.
Bar-Keep!! Drinks on the house!