I did a corned beef flat a month or so ago, although I had never done one before and didn't know smoking a a corned beef flat results in pastrami!
It was a 3.35 pounder I got at Wally World, preseasoned. I opened it, threw away the "seasoning packet" and, soaked for 48 hours and just rubbed with ground pepper
Paprika garlic powder ground mustard and resting in the fridge. (Changed the water 3 times during the soak)
started at 11:00 am lighting the coals. 250 degrees by 11:23 so on the brisket goes sitting in a foil pan. Internal temp is 43 (only rested out of the fridge 1/2 hour), dome temp dropped to 160 and grill temp at 124. By 11:38, dome temp is 280, grill is 295 and internal is 46. My plan was to take it to 155, add 1/2 cup of beer, then foil and bring the internal temp to 195-200 before pulling it and resting for 1/2 hour.
Grill temp kept moving between 290 and 305 until 1:24 pm. At 1:34 pm internal is 153 and grill is 297 (dome reads 270). For some reason my temp keeps dropping and I adjust the bottom vent slightly to keep it in the 295-300 range. Top vent is maybe 1/8" and bottom is about 1/4". Like I said earlier, if I can figure out how to upload pics from my iPhone I'll post.
At internal temp of 156 I added 1/2 cup of beer and covered pan with foil - 1:42pm. Grill temp dropped to 220 while messing with the beer and foil. By 1:45 grill temp back at 304 and internal temp at 160. Now at 2:15grill hovering between 293 and 297 degrees. Internal temp is 187. Tried the toothpick test at 195. Still not quite tender enough. At 2:49 pm Went to 208 and it was soft as butter. Preheated the oven to 250 then turned it off. Double wrapped in foil and let sit in the oven for 1 hour. Great results!