BTEC Grade calculator : No idea how to convert letters into numbers and print out final grade

Right, so basic problem. I'm designing a BTEC grade calculator, for those that are not in the know; BTEC is a type of qualification done in the UK and has a 'different' point grading system mainly based on coursework. There is a number of units (20) and each unit you can get a pass, merit and distinction on each unit. Each pass, merit and distinction add up to so many points and these points add up to the overall grade.

• BTEC Tariff Info
• DStar DStar DStar 420
• DStar DStar D 400
• DStar DD 380
• DDD 360
• DDM 320
• DMM 280
• MMM 240
• MMP 200
• MPP 160
• PPP 120

My problem is doing a mock up of a basic letter code print out is that with this BTEC system, I have no idea how to implement it; being a complete beginner at C# but knowing some basics.

Pseudo Code: Whats Your Name: - Name Enter your units: D M P D M P ( And the rest )

Your final grade is DMM

That's essentially what i need it to do is instead of putting 40 for a P I just want to put a P which would = 40 as such. This is my current code but it needs to be changed and fixed.

class Program
{
    static void Main(string[] args)
    {
        int counter;
        double score;

        while (true)
        {
            counter = 1;
            score = 0.0;
            Console.WriteLine("Enter Your Name or type 'exit' to quit): ");
            // Naming for exiting 
            string name = Console.ReadLine();
            if (name == "exit")
            {
                Environment.Exit(0);
            }
            else
            {
                int numberOfUnits = 20;
                Console.WriteLine("Number of Units:  ");
                numberOfUnits = int.Parse(Console.ReadLine());

                while (counter <= numberOfUnits)
                {
                    Console.WriteLine("Final Grade Score {0}", counter);
                    score += double.Parse(Console.ReadLine());
                    counter++;
                }
                // Add up and print letter grade. 
                score = (score / (counter - 1));
                if (score < 120)
                {
                    Console.WriteLine("Letter Grade: PPP");
                }
                else if (score < 160)
                {
                    Console.WriteLine("Letter Grade: MPP");
                }
                else if (score < 200)
                {
                    Console.WriteLine("Letter Grade: MMP");
                }
                else if (score < 240)
                {
                    Console.WriteLine("Letter Grade: MMM");
                }
                else if (score < 280)
                {
                    Console.WriteLine("Letter Grade: DMM");
                }
                else if (score < 320)
                {
                    Console.WriteLine("Letter Grade: DDM");
                }
                else if (score < 360)
                {
                    Console.WriteLine("Letter Grade: DDD");
                }
                    // After DDD it goes up by 20's ..... D* = 20 not 40...
                else if (score < 380)
                {
                    Console.WriteLine("Letter Grade: D*DD");
                }
                else if (score < 400)
                {
                    Console.WriteLine("Letter Grade: D*D*D");
                }
                else if (score < 420)
                {
                    Console.WriteLine("Letter Grade: D*D*D*");
                }
                Console.WriteLine("Grade: {0}", (score).ToString("P"));
            }
        }
    }
}

}