I Know that his resturant pits run between 275 & 325 and I don't remember seeing a thermometer except on the pit.
A WSM has the heat coming from below to me that means fat cap down.
If you want something that is as close to what he cooks you need to change your Game. I can get real close and I cook select & choice grade meat From Wal mart & HEB.
Since your cooking on a WSM this is how I cook on the UDS( NO waterpan no diffuser)
cook temp 300
trim the hard fat
S&P
FC down 4 hrs
Wrap in BP
Continue at 300
1-2 HRs it Will probe tender in the thickest part of the Flat.
let it rest wrapped on the counter top until the internal temp drops into the 150's
never fails grade don't matter weight don't matter
6 hrs cook and aprox 2 hr rest
it jiggles when it hits the board and melts in yo mouf