NOTE: This method is specifically designed for smaller prime ribs, under 8 pounds. It is also calibrated to produce a perfect, medium-rare slice of beef. If you're doing a larger prime rib, or want it cooked more, check out this traditional method for prime rib of beef au jus.
Buy a "bone-in" prime rib of beef roast. They are usually trimmed for the oven and will look like the photo. If it still has a large thick layer of fat on top, trim this off - leaving only a 1/4-inch to 1/2-inch layer.
Place the prime rib in a heavy roasting pan, and leave out to come up to room temperature. This is a CRITICAL step. If the prime rib is ice-cold the time formula will not be accurate. You should leave it out for 2-4 hours depending on the size - some even leave it out, wrapped, over-night.