Harry's letter begins with a series of numbers:
1, 2, 4, 6, 8, 11, 16, 16, 17, 17, 18, 20

To find The Magic Quantile, given a sequence of numbers, the platform where we can get on the train to Hogwarts is the Q3 value of the data. (Make sure to calculate the best possible approximation of Q3 if it is not an exact number.)

(In other words, just find Q3)