The Quantile() function will give you what you want if you are looking to calculate your percentiles for an array of data.
names default to here(1);
A = [1.2, 1.5, 10, 25, 31, 40, 50, 99, 1000, 5000, 25000, 100000];
P50 = Quantile( .5, A );
This can also be used against a column of data in a data table if you reference the data table as a matrix of data
names default to here(1);
dt = open("$SAMPLE_DATA/Big Class.jmp");
P75 = Quantile( .75, dt[0,4]);
Jim