The first thing you need to determine when cooking a standing rib roast is just how much you want it cooked. For a standing rib roast an internal temperature of 125 degrees F. (52 degrees C.) is about rare and 145 degrees F. (63 degrees C.) is medium. If you want it cooked more than this, consider cooking something else. A standing rib roast is best around medium rare or about 135 degrees F. (57 degrees C.). To think about this properly you want to consider the cooking backwards. A standing rib roast should sit, loosely covered with foil in a warm place for about 20 minutes. This will allow the juices in the roast to seep back into place. However the roast will continue to cook from its own heat during this time. This will give you about 10 more degrees F.
So, if your target temperature was about 135 degrees F. then you will want to pull your roast out of the oven at around 125 degrees F. Now, to get that great crust over the surface of the meat you will want to put your standing rib roast in the oven. Preheat the oven to 400 degrees F. (205 degrees C.). When you put the roast in the oven, off the heat. In the time (about 20 minutes) it takes to make the crust your rib roast will gain about 15 degrees F. So if your target is 135 degrees F. you will want to take your roast off the smoker when its internal temperature is about 110 degrees F. (43 degrees C.). Put it in a roasting pan on a rack so it doesn't sit directly in its own juices. Place the roast in the preheated oven and turn off the heat. Leave it there for about 15 to 20 minutes or until the internal temperature reaches 125 degrees F. Don't open the oven door during this time.
So what we need to get to this point is a standing rib roast cooked to an internal temperature of about 110 degrees F. (assuming your target is 135 degrees F.) You get to this temperature in the smoker. Your smoker temperature should be about 225 degrees F. (107 degrees C.) for cooking your rib roast. Since this is one of the tenderest pieces of meat you are going to find you won't need to be as concerned with the temperature as you would with say a beef brisket but keeping to this range will help you plan on your cooking time. You should plan on about 30 minutes per pound in the smoker.
A standing rib roast should be smoked without a lot of smoke. While you want to add flavor to your roast you don't want to overpower it. Stick with a mellow wood like an oak or fruit wood and use it in small amounts. Remember you can add more smoke, but you can't take it out. Other than that you should have no trouble smoking your rib roast. Keep an eye on your thermometer and when it reaches the desired temperature get it out of the smoker.
Now we get back to the beginning of the process. When smoking a rib roast in this way you want to start with a roast that is close to room temperature. I suggest getting the rib roast out of your refrigerator 2 hours before it goes in the smoker. Season it as desired (a simple rub is best), cover loosely with foil and let it sit on the counter out of the way. This will slowly raise the temperature. Normally a rib roast will be about 35 to 40 degrees F. in your refrigerator. Letting it warm up to about 50 degrees F. before you start smoking will reduce the total cooking time and put you in the more productive smoking temperature.
Using this technique a standing rib roast weighing in around 8 pounds should take you about 4 to 5 hours to cook. You can adjust the various times as needed to get your roast done on time. For instance if you find that your roast isn't up to temperature in the smoker let it spend more time in the oven. But remember to watch those temperatures very closely. When it comes to getting this piece of meat cooked to perfection it isn't a matter of time as much as it is a matter of temperature.