Here's a never fail Prime Rib Recipe... works for any size Prime Rib... I have done it using a roast with 8 ribs...
I also like to add peeled whole onions to the roasting pan during cooking...

[color:"blue"]Roast Prime Rib of Beef Recipe[/color]

[color:"blue"]INGREDIENTS[/color]
4-10+ pound rib roast
1 cup beef broth
S&P to taste
Garlic slivers inserted with knife (optional)
Kitchen Bouquet to rub on outside of roast

[color:"blue"]DIRECTIONS[/color]
1. Have your butcher trim the short ribs from the roast. Place the meat in a dish and let it come to room temperature. Rub all sides with salt and pepper and then Kitchen Bouquet.
2. make small slits in meat with boning knife and insert garlic slivers (optional)
3. Place the meat on a rack in a shallow roasting pan.
4. Roast in a preheated 500 degree F oven for 20 minutes.
5. Reduce the heat to 325 degree F and roast 2 hours longer for medium-rare.
6. [color:"red"](DO NOT OPEN OVEN DOOR UNTIL TIME IS FINISHED, or internal temp reaches 115F!!!)[/color]
7. Transfer the roast to a heated platter.
8. Cover loosely and let stand for 15 minutes before carving.
9. For the Au Jus: Spoon off any excess fat from the roasting pan.
10. Measure out 1/2 cup of the beef juices and reserve for the Yorkshire pudding.
11. Pour the beef broth into the roasting pan.
12. Bring to a simmer over moderately high heat, scraping up any brown bits clinging to the bottom and sides of the pan.
13. Season with salt and pepper.
14. Strain the roasting juices into a heated sauce boat and serve with the roast.

[color:"red"]NOTE:[/color] I use a wired remote cooking thermometer until internal temp is 115F...
it will go up to about 120F-125F during the resting period...