I'm doing a 14.5 lb'er myself today and have done this several times before. I'll cook mine at 325-350 cooker temp until it reaches 120 IT, then I'll either put it on another HOT smoker (450-500d) or likely just throw it in the oven at 500d until the IT is at 135 (med rare). Then I rest and carve. I'm planning on about 4 hours total cook time, including the "reverse sear".
Here's a good website on the basics of making a prime rib roast:
NOTE: although the website is not necessarily about SMOKING, times and temps and basics will still apply.