Hi Rich,
My first brisket was also a flat from Costco!
It turned out alright, but I made some mistakes that kept it from being "very good" or better. I actually posted some photos and process info. If you are interested in seeing a few things
not to do, you can go here -
http://tvwbb.com/showthread.php?61255-Advice-on-Cooking-Brisket".
Learned a lot in the last eights months. Below are some things I have found for cooking flats. Please keep in mind that these are just my opinions. Some people may disagree. Feel free to ignore anything or everything.
- Most important: Like Tim said, go by feel, not just internal temperature! Early on, I was so obsessed with temps and measurements that I was taking the briskets off too early. Use the meat temp around 185 or 190 as a starting point to check the feel. For a flat, the probe should feel like it's going through crisco or watermelon. The meat should feel very "bendy" and supple.
- Second most important: let it rest! Put it in a cooler for 1 to 2 hours. This makes a big difference for me.
- Wrap the flat earlier than you normally would. I agree with James - wrap around 160.
- Regarding wrap: I have tried not wrapping at all, as well as wrapping with foil and butcher paper. I prefer butcher paper but foil is pretty good. Just don't add any liquid to the wrap! I tried that a few times and I got pot roast.
- Salt liberally the night before. Then add just pepper when you are ready to cook.
- In your photo there is not much fat so don't trim too much, if any. I like to leave a bigger fat cap on flats. You don't have to eat it, but it helps.
- I like to use a clay saucer instead of water.
- Last but not least: low and sloooow makes the best flats, IMO. The slower, the better (within reason). I keep the WSM at about 210 for brisket flats.
Again, just some opinions from my trial and error.