The cooking time depends on how big your rack of lamb is, and how rare you want it cooked. Rack of lamb should be cooked rare, or at most medium rare. The instructions are for a rack 1½ to 2 pounds big. If you have a smaller roast (I've seen packages of just a pound), reduce the cooking time from 7 minutes to 5 minutes on 400, and use the lower end of the cooking time given at 300. If you are cooking multiple racks (unless you are doing a crown roast which is a different matter), lay them out separately on the pan, and estimate a starting roasting time based on just one rack, not the weight of the combined. Though you may need to add some time to the overall cooking time if cooking multiple racks. Of course, use a meat thermometer! This is too lovely and tender a roast to risk overcooking.