Tiger I have done flats maybe 6 times and only one that was in the 4lb range. For that smoke I put it in a pan, this way it did not dry. Below is a recipe that has worked great for flats that are more in the 6-8 lb range.
But I would suggest buying a whole brisket it is cheaper and it will turn out much better. Also use a water pan as others have said and make sure the brisket is really done, temp can be any ware from 194-200 range. Also if you can find a prime brisket (all I use now) you will never go back to a choice cut of meat. Check your local Costco, normally they have it for about $4.00 a lb.
Best Greg
Brisket
6.7 lb. choice flat from Costco
Wood:
5.5 oz. Pecan
Quick Tender:
Apply and allow to sit for 30 minutes, rinse well and then pat dry & apply rub
Rub:
3 T Pickle Juice/3 T Yellow Mustard
Rub 7 Parts Black Pepper, 3 Parts Kosher Salt, 3 Parts Lawry’s, 1 Part Garlic
Directions:
Apply yellow mustard and then the rub, place in refrigerator overnight
Place in smoker at 260 degrees with the fat side down.
Once it hits 195 degrees test to make sure it is tender, remove from smoker. Place the brisket in pan covered with foil (add 1 cup Au Jus) place back in the smoker at 160 degrees, until dinner time.
* Took 7.5 hours to come up to 195 degrees, excellent brisket.