Roast should be at room temperature before roasting in the oven. Prepare the roast and allow it to sit in the pan with the seasoning on it for about 20 minutes or more before roasting.
Don’t add water or any liquid to the pan.
Letting the roast rest after cooking before slicing allows it to retain its juices.
A bone-in roast may need a little more cooking time so be sure to check the internal temperature after 1 hour of roasting.
An instant-read digital meat thermometer makes it easy to get a fast and accurate reading on the meat temperature.
I think a prime rib is best served Medium Rare to Medium. I think anything over 160°F becomes dry and very over cooked, but here is the general rule.
My general rule for Internal Cooking Temperatures
1) Rare – Bright Red Center -- About 140°F, 63°C
2) Medium Rare – Medium – About 145°F, 63°C
3) Medium – Well – About 160°F, 71°C
Keep in mind that oven temperatures can vary, so if you think your oven is not accurate, check the temperature by using an oven-safe thermometer on the center rack in the oven and heat the oven for 15 minutes. Then check to see that your oven is reading the same temperature that it was set at. If it is off, you will need to adjust your cooking time and temperature accordingly.