using System;

​

public class Program

{

    public static void Main()

    {

        decimal inputValue = 90m;

        int precision = -1;

        var interval = GetInterval(inputValue, precision);

        System.Console.WriteLine("first: " + inputValue + ", precision: " + precision + ". low: " + interval.low + " . high: " + interval.high);

        inputValue = 85m;

        precision = 0;

        interval = GetInterval(inputValue, precision);

        System.Console.WriteLine("first: " + inputValue + ", precision: " + precision + ". low: " + interval.low + " . high: " + interval.high);

        inputValue = 85.5m;

        precision = 1;

        interval = GetInterval(inputValue, precision);

        System.Console.WriteLine("first: " + inputValue + ", precision: " + precision + ". low: " + interval.low + " . high: " + interval.high);        

    }

    public static Interval GetInterval(decimal baseValue, int precision)

    {

        decimal intervalWhole = 1 / (decimal) Math.Pow(10, precision);

        decimal intervalHalf = intervalWhole / 2;

        var lowValue = baseValue - intervalHalf;

        var highValue = baseValue + intervalHalf;

        return new Interval(){low = lowValue, high = highValue};

    }

    public struct Interval 

    {

        public decimal low;

        public decimal high;

    }

}