I have a few data points that you might want to take with several grains of salt. It looks like your son took the 2nd grade test as a 1st grader, is that right? If so, that means that the percentiles will be a bit more forgiving, which is good.
Here is what I have:
When my son took the level 7 test, he missed one question in the reading comprehension section. The percentile rank was 91 as compared to 1st graders in the spring and 99 as compared to 1st graders in the fall.
When he took the level 8 test, he missed two questions in the RC section, which gave him a PR of 92 as compared to 2nd graders. I don't have what it was as compared to 1st graders--which is probably what you need.
When he took the level 11 test, he missed 3 questions on the RC section, with a PR of 93 as compared to 5th graders, and 99 as compared to 3rd graders.
