My feeling for a long time has been that mastery/expertise with a particular grade level's material is indicated by a percentile rank of 90 and above. I based this on my knowledge of what is supposed to be taught at various grade levels, what is actually on the tests, and how my children have done on various tests when I am sure of their level of mastery.
But until a few months ago I had never seen anything official that confirmed this until I found this in the Iowa Acceleration Scale Manual (3rd edition): "In assessing achievement as it relates to making a decision about acceleration, earning a grade-level score that results in a ranking in the 90th-94th percentile indicates mastery (achievement) of the grade-level content."
So for making placement decisions, I would use the 90th percentile and above on an out of level test.
