Calculate Percentile

Main.cs

void Main()
{
    var data = new double[] { 2, 4, 6, 8, 13, 16, 22, 35, 40, 42, 48 };
    var percs = new[] { 0.5, 0.66, 0.75, 0.80, 0.90, 0.95, 0.98, 0.99, 1.00 };
    percs.Select(x => 
    {
        var percent = (int)(x * 100);
        var percentile = MathUtil.Percentile(data, x);
        var percentileInterpolation = MathUtil.PercentileInterpolation(data, x);
        return new Result(percent, percentile, percentileInterpolation);
    })
    .Dump();
}

public struct Result
{
    public int Percent { get; set; }
    public double Percentile { get; set; }
    public double PercentileInterpolation { get; set; }
    public Result(int percent, double percentile, double percentileInterpolation)
    {
        Percent = percent;
        Percentile = percentile;
        PercentileInterpolation = percentileInterpolation;
    }
}

Percentile.cs

public static class MathUtil
{
    public static double Percentile(double[] sortedSequence, double percentile)
    {
        var n = sortedSequence.Length;
        var realIndex = Round((n - 1) * percentile);
        var rank = (int)realIndex;
        var flac = realIndex - rank;
        if (rank + 1 < n)
        {
            return Round(sortedSequence[rank] * (1 - flac) + sortedSequence[rank + 1] * flac);
        }
        else
        {
            return sortedSequence[rank];
        }
    }

    public static double PercentileInterpolation(double[] sortedSequence, double percentile)
    {
        var n = sortedSequence.Length;
        var realIndex = Round((n + 1) * percentile) - 1;
        var rank = (int)realIndex;
        var flac = realIndex - rank;
        if (rank >= n)
        {
            // last
            return sortedSequence[n - 1];
        }
        else if (flac == 0)
        {
            // when index match to rank
            return sortedSequence[rank];
        }
        else if (rank + 1 < n)
        {
            // calculate interpolation
            return Round(sortedSequence[rank] + (sortedSequence[rank + 1] - sortedSequence[rank]) * flac);
        }
        else
        {
            return sortedSequence[rank];
        }
    }

    private static double Round(double value, int digit = 2)
    {
        return System.Math.Round(value, digit, MidpointRounding.AwayFromZero);
    }
}

Result.md

Percent	Percentile	PercentileInterpolation
50	16	16
66	29.8	33.96
75	37.5	40
80	40	41.2
90	42	46.8
95	45	48
98	46.8	48
99	47.4	48
100	48	48
753	352.5	369.96