Anything below 185 internal and you'll have a tough to chew brisket.
The best way to know is to use the internal temp as a guide, not a rule, as the fat content will have an effect on the brisket as it cooks and the collagen does it's magic.
I use a pencil thermometer to check various spots inside the brisket for temp. The outer ends will have a different temp then the internal. Now, the key, then is as you push the probe in, how much give does it have? If the probe doesn't go in easy, the brisket isn't done. Practice and keep good notes.
As for marinades, rubs, injections, etc. Everyone has a theory and mine is to go with a simple brisket until you get it how you like. The start with marinades or whatever. Briskets in Texas are simple, they add a rub of salt, pepper, maybe garlic and the smoke. If you can smoke a brisket that way, you're doing well.