I have a SM066 as well and did a large prime brisket like you are doing a few weeks ago. I set the smoker to 230 degrees and the probe to 188. I don't recall exactly but I think it took 15 to 16 hours and the flat was perfect after two hours of FTC. Separated the point, reseasoned and dropped it back in the 066 for two more hours for excellent burnt ends. I just did a much smaller one a couple days ago (I think around 12 pounds) and it finished in about 11 hours.