Rob... We have had standing rib roast for Christmas for the last 35 years but last year I cooked using my WSM for the first time. We prefer to cook 2 smaller roast since my sons love it rare and the rest of the family want it medium rare. My immediate family consist of 10 adults, 3 teenagers and 2 grade school age girls, so I prepare about 20 lbs.
Why don't you place the roast in a pan to save the juices? I use a roasting pan and place the meat on a rack. Works everytime for me...
Good luck and have A MERRY CHRISTMAS!
joe