Which part of a rib roast has the most of the cap?
You are on the right track.
Thr rib-eye cap or Spinalis Dorsi muscle is largest between ribs 8-10. To maximize your cap, you may want to order 2 roasts that include ribs 8-10 rather than order 1 large roast (ribs 6-12).
For more info regarding Spinalis Dorsi, refer to the posts by "FlyFish" in the following thread: