Writing a draft of your own recommendation letter: how to compare yourself to other students?
Write the letter the way you want it written.  Send it to your recommender along with a note that says something like


  I've been offered advice to the effect that the best letters offer a
  direct comparison to a peer group.  Toward this end, I've provided
  some recommended language comparing me to the group I feel is most
  relevant for this letter in paragraph X.  Obviously, I'm not perfectly
  comfortable doing this, and I suggest that you read that paragraph
  carefully and adjust it for accuracy.