using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var rnd = new Random();

        var percentage = (decimal)rnd.Next(100);

        percentage += (decimal)rnd.Next(100) / 100;

        var listOfStrings = new List<string>

        {

            "Test1",

            "Test2",

            "Test3",

            "Test4",

            "Test5",

            "Test6",

            "Control",

        };

        var index = GetListIndexFromPercentage(percentage, listOfStrings);

        Console.WriteLine(percentage + " = " + index);

    }

    private static int GetListIndexFromPercentage(decimal percentage, List<string> list)

    {

        // 1 = 100

        // 2 = 50

        // 3 = 33,3333333

        // 4 = 25

        // 5 = 20

        // ...

        var modifier = 100m / list.Count;

        Console.WriteLine($"Modifier: {modifier}");

        var result = percentage / modifier;

        Console.WriteLine($"Result: {result}");

        return (int)Math.Floor(result);

    }

}