Smoking my first brisket

My advice would be cook to internal temp and use that as a starting point for probing with a skewer. When the probe goes right in nice and easy you are good. I've learned that the brisket is done when it's done not when my timer says so.

Not sure if you already did your brisket, but if so, please share some pics or let us know how it turned out!

12 hours seems a little short for a brisket, then again, sounds like you are only doing a couple flats, so if you wrap you can probably turn them out in that amount of time.

What sort of smoker are you using? Are you wrapping or no? Are you injecting?

About your injecting question, I used to have the opinion that somehow injecting is "less honorable" or whatever you want to call it, and a lot of people on here will get all high and mighty about letting the meat do the talking or whatever else, but not all of us can afford Waygu or even Prime cuts, so injecting can really help out the meat, especially if you keep it simple. I went to a class with Diva Q, and a few local competition teams and all of them said they inject 100% of the time in our out of competition, even when they use a Waygu, or other high end cut.

After that I started injecting 100% of the time (I use Butcher BBQ Prime Brisket Injection) and my meat has been significantly more tender, and tastier. Their injections doesn't overpower the meat, its mostly just a broth, and I don't inject even half as much as the instructions say too.

If BBQ is all about taking less desirable cuts of meat and doing whatever you can to make them amazing, I've got no problem injecting. I struggled with Briskets for a long time, getting great ones sometimes, and dry one other times doing the exact same methods, (including the toothpick test mentioned above) and it wasn't till I started injecting that I've been able to turn out solidly juicy brisket every time (part of this is thanks to the phosphate aspect of a commercial injection like Butcher BBQ, that like you juicy/tender insurance policy, and why not have a good policy on such an expensive piece of meat).

Just be careful with your Broth selection, I can tell you as a KCBS judge that I can tell the ones where they use too much, or one with higher sodium, or especially when they add to much Worseshire (which is a common addition to a broth injection).