I've done a two and three 'bone in' prime rib, indirect couple times. Never the whole roast. Which I'd like to try for my dad's 80th birthday party Saturday. Usually I've seared the roast 4 minutes on both sides at 700 degrees, take it off and stabilize heat down to 325 then let her rip for about hour and half or until center temp reaches 145 for medium rare. So my question is I've done 5 to 6lb roast's like this and usually takes about hour half, how long will it take for a whole 7 or eight bone in prime rib roast?