using System;

using System.Collections.Generic;

public class Program

{

    static double RoundBasedOnCustomThreshold(double number, double customThreshold = 0.25)

    {

        // customThreshold of 1 will be equivalent to Math.Floor

        if (customThreshold <= 0 || customThreshold > 1)

            throw new ArgumentException();

​

        return Math.Round(number + 0.5 - customThreshold, 0, MidpointRounding.AwayFromZero);

    }

​

    public static void Main()

    {

        Console.WriteLine(RoundBasedOnCustomThreshold(6.75, 0.75));

        Console.WriteLine(RoundBasedOnCustomThreshold(6.25, 0.25));

        Console.WriteLine(RoundBasedOnCustomThreshold(6.5, 0.5));

        Console.WriteLine(RoundBasedOnCustomThreshold(6.5, 1)); 

        Console.WriteLine(RoundBasedOnCustomThreshold(6, 1));

        Console.WriteLine(RoundBasedOnCustomThreshold(6, 0.5));

        Console.WriteLine(RoundBasedOnCustomThreshold(6, 0.9999));

        Console.WriteLine(RoundBasedOnCustomThreshold(5.9999, 0.9999));

    }

}