Помогите, понять, как считается 90 перцентиль из набора чисел.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Percentile {
    public static void main(String[] args) {

        ArrayList<Double> doubleArrayList = new ArrayList<>();
        File file = new File("C:\\java\\12344.txt");
        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = reader.readLine()) != null) {
                doubleArrayList.add(Double.valueOf(line));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.printf("%.2f%n", percentile(doubleArrayList, 90));
    }

    public static double percentile(List<Double> latencies, double percentile) {
        Collections.sort(latencies);
        int index = (int) Math.ceil(percentile / 100.0 * latencies.size());
        return latencies.get(index - 1);
    }

}