using System;

public class Program

{

    public static void Main()

    {

        var values = new int[] { 100, 500, 1000, 5000, 100000, 500000 };

        foreach(var n in values)

            Console.WriteLine($"{n} => {TickRange(n)}");

    }

    private static int TickRange(int maxValue)

    {

        // get the numerical value of the first digit. Is there a better way to do this?

        var digit = maxValue / Math.Pow(10, (int)Math.Log10(maxValue));

​

        // some magic to get the proper exponent 

        var exp = Math.Pow(10, (int)Math.Log10(maxValue / 5)  - 1);

        return (int)((digit < 5 ? 5 : 1) * exp);

    }

}