I've done several on the bbq wrapped in foil or not wrapped on the top rack. I put a prime rib spice and a little montreal steak spice on then vac seal and let sit a day before cooking. Take it out of the fridge and let it warm up some or you will have well done on the outside and rare/very rare in the middle. I'd say around 20-25 minutes per pound the bigger the longer. I use a meat thermometer to judge when it's done.
