Do the PDFs on this page help?

http://erblearn.org/schools/collaboration/quality-assessment/norms-and-scoring

See, e.g., the table on p. 95 of the spring book.

One thing I was surprised by was where the bottom of the 99th percentile of the national norm matched up with the suburban and independent school norm. In math especially, the norms imply that 10-20% of the kids in independent schools would be in the top 1% nationally.