my district has informed me that there is no ceiling on this test. the only ceiling you would have would be concepts not yet introduced. DD7, last year, got caught up in area and perimeter. He was tested on them and the school happily told me that he got them wrong. I asked if the concept had been introduced, they said no. so I explained to him , once, what perimeter is, tried to show him on Khan Academy (he wasn't interested, I got it mom) and can answer it correctly.
here is a sample DesCartes page that I found which should help you assess the 50% proficiency range associated with the numbers correlating on the MAP results.
http://www.wall.k12.nj.us/map/documents/RIT_Math.pdf
