3/4 chimney should be more than ample fuel for what you are doing. Not sure that adding more buys anything, but I don't use the rotisserie mod either. I start with less than 3/4. Rub down fully trimmed tri-tips with a good quality rub several hours in advance. All of the coals poured to one side of the kettle plus a couple small chunks of mesquite, oak, or hickory along the outer edges of the lit charcoal. Sear the meat well over the hot coals and smoldering wood, and cook indirect under the lid approx 30 minutes, turning once during the process. You will see more barkening on the first turn and the juices just wanting to piss everywhere. Pull after about 30 minutes (15 minutes after the turn), wrap in foil and rest for another 10 minutes before slicing. Perfect.