Okay I'll jump in. There are so many factors to consider, this question is difficult to answer but I'll try. Long but hopefully helpful!
Brisket is the hardest to cook well for a reason - depending on the cut your using you have differing amounts of marbling and different thicknesses. A choice or CAB brisket bought at Sam's for instance is usually a whole packer with the point (and fat layer between) removed then cut in half - there is very little fat in this cut to keep things moist through a long cook. A big old well marbled 13 to 15# whole packer is still difficult to cook well but I think a little more forgiving because of the fat content.
We learned the hard way cooking too low and too slow and ended up with dry brisket every time. We started cranking up the temps even trying "fast & hot" a couple times with success but have settled into a successful groove keeping grate temp near but not below 250 with 275 as a high point.
Cook temp and time is also rather smoker specific. Like Pop said, you could probably get away with that low a temp (but why) if you had a really tight smoker like a Jambo that stayed at temp with very little air flow. Running a el cheapo leaky offset (high air flow) at a low temp will dry out a brisket in no time. On our 22 WSM's (which I would rank a 7 out of 10 for tightness) we use full simmering water pans so we have a lot of moisture in the system (which helps immensely) with moderate air flow.
So Moda a quick guideline to try (if you haven't already given it a go) with the cut you mention - sounds like a flat. Fire and smoke - good starting point for smoke level - embed 2 or 3 fist size chunks of your favorite hard/fruit wood in pile of unlit charcoal - 1 starter of lit on top - allow to burn in and spark off any dust, white smoke should die down signaling a clean burn - 2 to 3 chunks on top right before placing grate - top vent should always remain full open - bottom vents full open to start. Get a cheap oven thermometer and place it in the middle of the grate. Preheat your UDS to a grate temp of 250 (if you place it in the right spot after your meat is placed you can use a flashlight to see the grate temp through the top vent w/o opening) adjusting bottom vents as needed. Your spice rub blend is actually very traditional and simple and should work nicely - injecting beef broth can't hurt either but I would let the meat rest a bit after injecting so it can distribute then add the rub which with the injection becomes more of a wet rub - will dissolve in nicely into meat. I typically massage in one layer of rub then add a second sprinkled from a shaker so the rub sits proud of the meat - helps create a nice bark. I am a big fan of water smokers and water pans so I would recommend if your not using one in your UDS to use an aluminum pan full of water. Fill it back up when you change/add charcoal. Not going to give you times but we find at 250 in our WSM's BB is about an hour a pound pulling at 195 in the thickest part of the flat. Don't worry about probing temp for a few hours just let it cook and no peaking - hopefully you can see your grate thermo. After 3 or 4 hours take a look at where your color is and bark formation - go ahead and insert a leave-in meat thermo or probe. The finished color I like is a deep mahogany verging on black with streaks that are almost golden in color. My technique requires tenting loosely before this color is reached because I open it back up for the last hour to set the bark. Unless you have hungry guests and need to speed things up or just can't seem to maintain moisture I don't foil tightly. If you have large enough foil pans (otherwise make a boat out of HDAF) place the meat in the pan which will capture juices then loosely tent with HDAF to hold the color and smoke level. Keep cooking! If your not sure when your about an hour out then pull the tent around 185 internal - leave in pan. The spice blend will mellow over the length of the cook so if you like a nice salty peppery kick balanced by a nice beefy interior then sprinkle a modest third layer of your rub right after pulling the tent. Cook until an internal of 195 (temp will rise another 5 degrees or so while resting. If serving immediately place on a butcher block and let rest for 10 to 15 minutes in the open before slicing and serving. If you need to hold a little longer you can again tent loosely - don't slice until your ready to serve! The drippings are full of flavor and depending on how much rendered can be used straight or added to broth to make an au jus. Enjoy!
Disclaimer: Some portions or steps of this technique have been withheld to protect the innocent aka the above I think is a good starting point. Additions, subtractions, thoughts are certainly welcome as the learning process is never-ending.
Good luck with your first brisket. Let us know how it turns/turned out!
Matt Schneider - "The Smokesman" - Velvet Smoke BBQ & Catering
KCBS Certified BBQ Judge #54767
Weber Smokey Mountain 22 & Superior Smoker SS1
[B][COLOR="Red"]"To BBQ is human, for swine is divine!"[/COLOR][/B]