using System;

using System.Collections.Generic; 

​

public class Program

{

    static int ToConfigValueFromDisplay<T>(T displayValue, IList<T> values){

        var index = values.IndexOf(displayValue);

        var percent = ((double) index) / (values.Count - 1);

        return (int)Math.Round(percent*100);

    }

    static int PercentageToIndex<T>(IList<T> list, int percentage){

        var percentToDecimal = (double) percentage / 100.0;

        var index = percentToDecimal * (list.Count - 1);

        return (int)Math.Round(index);

    }

    public static void Main()

    {

        var ui = new List<string>(){"one","two","three","four","five","six","seven","eight","nine"};

        Console.WriteLine(Program.ToConfigValueFromDisplay("one",ui)); //should see 0

        Console.WriteLine(Program.ToConfigValueFromDisplay("two",ui)); //should see 12

        Console.WriteLine(Program.ToConfigValueFromDisplay("three",ui)); //should see 25

        Console.WriteLine(Program.ToConfigValueFromDisplay("four",ui)); //should see 37

        Console.WriteLine(Program.ToConfigValueFromDisplay("five",ui)); //should see 50

        Console.WriteLine(Program.ToConfigValueFromDisplay("six",ui)); //should see 62

        Console.WriteLine(Program.ToConfigValueFromDisplay("seven",ui)); //should see 75

        Console.WriteLine(Program.ToConfigValueFromDisplay("eight",ui)); //should see 87

        Console.WriteLine(Program.ToConfigValueFromDisplay("nine",ui)); //should see 100

        Console.WriteLine(Program.PercentageToIndex(ui,0)); //should see 0

        Console.WriteLine(Program.PercentageToIndex(ui,12)); //should see 1

        Console.WriteLine(Program.PercentageToIndex(ui,25)); //should see 2

        Console.WriteLine(Program.PercentageToIndex(ui,37)); //should see 3

        Console.WriteLine(Program.PercentageToIndex(ui,50)); //should see 4

        Console.WriteLine(Program.PercentageToIndex(ui,62)); //should see 5

        Console.WriteLine(Program.PercentageToIndex(ui,75)); //should see 6

        Console.WriteLine(Program.PercentageToIndex(ui,87)); //should see 7

        Console.WriteLine(Program.PercentageToIndex(ui,100)); //should see 8

    }

}