Rit scores do not correlate with a grade. They are giving the Map-P to make sure your child is showing growth through the years. The score you received is just a baseline and states that your child scored equal to or better than 85% of children nationwide- which is fabulous! It does not analyze just reading at this stage as all k-2 children wear headphones and the questions are read to them.
In my current 4th grade class, I have 17 students out of 24 that scored above the 90th percentile and all students scored above the 75th. We don't worry about the percent, and just check to see how much growth they had since the previous year and see what areas can still be improved.
Your child will take the test 3 times a year. Concern yourself with checking to see if your child can improve over time. Your child is off to a great start!