Are we talking a butt or a picnic? The butts I buy don't have a large fat cap on them.
Regarding the picnic, I like to separate the fat cap under the skin from the meat: 1) by trimming it back with a filleting knife to the ankle, 2) applying my rub underneath, 3) pulling the loosened fat cap back over the rubbed picnic, 4) refrigerate over night, 5) put it in the CS @225 fat cap up, and 6) sit back and wait 18 hours (195 degrees) or so.
Happy Trails