I trim the "fat cap" off totally.
Keeps it simple.
Butts have so much internal fat, they cook up just fine without the fat cap.
And, rub don't stick to fat--so I get killer bark all around.
Fact is, I trim out some of the major internal fat areas for even more bark.
Judges like it, and so do I.
Works for me.
Your mileage may vary.
"Flirtin' with Disaster" BBQ Team (RETIRED)
FBA and KCBS Cook and Judge.
Former owner of a WSM, a Smokey Joe, a Charbroil Commercial gasser (junk), the legendary "StudeDera", a Fast Eddy PG500, and Sherman.
Now cooking with a Yoder YS640
Proud Pellet guy cooking on real wood.