Use a meat thermometer so that you can watch the internal temperature of the meat. Roast your meat at 325 degrees until the internal temperature reaches 130 degrees (this will be medium rare). Keep in mind that the meat will continue to cook after you take if off the grill. For a 6 lb rib roast, this will take about 1.5-2 hours.
I recommend using the Thermapen to check internal temps as a full-proof method for measuring temperature.