C # - 5 significant numbers with at least 2 decimal places

Well, someone knows a way that I can transform a number with at least 5 significant digits and added to this factor, should have at least 2 decimal places.

Example:

-0,00012 >> -0,00012000
12345687,00 >> 12345687,00
123,00 >> 123,00
123,21254 >> 123,21

NOTE: Significant figure starts counting from the first non-zero number, from left to right

I came to this function, but I'm sure to improve it, I ran!

        static string RoundToSignificantDigits(decimal d)
    {



        string numero = d.ToString();

        if (numero.IndexOf(",") == -1) numero += ",00";

        int signif = 0;
        int index = 0;
        bool startCount = false;
        for (int i = 0; i < numero.Length; i++)
        {
            index = i;

            if (!startCount && numero[i] != '0' && numero[i] != ',' && numero[i] != '-')
            {
                startCount = true;
            }

            if (numero[0] == '0' && i == 0)
            {
                i = 2;
            }

            if (numero[i] != ',' && numero[i] != '-')
            {
                if (startCount)
                    signif++;
            }

            if (signif == 5)
                break;
        }

        int alg = d < 0 ? 1 : 0;

        if ((d>0.001m &&index == 4) || (index > numero.IndexOf(",") && numero[alg] != '0'))
        {
            string[] parts0 = numero.Split(',');
            parts0[1] = parts0[1].Substring(0, 2);
            numero = string.Join(",", parts0);

            if (index<parts0[0].Length || signif ==5)
            {
                return numero;
            }

        }
        else if (numero[alg] == '0')
        {
            numero = numero.Substring(0, index + 1);
        }
        else
        {
            string[] parts = numero.Split(',');
            parts[1] = parts[1].PadRight(2, '0');
            numero = string.Join(",", parts);

            return numero;
        }



        if (numero.IndexOf("-") != -1)
        {
            if (d < 0)
            {
                string[] parts = numero.Split(',');
                int numbers = parts[1].TrimStart('0').Length;
                int pad0 = 5 - numbers;

                if (pad0 > 0)
                {
                    parts[1] = parts[1].PadRight(parts[1].Length + pad0, '0');
                }

                numero = string.Join(",", parts);

            }
            else
            {
                numero = numero.PadRight(7, '0');
            }
        }
        else
        {
            string[] parts = numero.Split(',');
            int numbers = parts[1].TrimStart('0').Length;
            int pad0 = 5 - numbers;

            if (pad0 > 0 && pad0 != 5)
            {
                parts[1] = parts[1].PadRight(parts[1].Length + pad0, '0');
            }

            numero = string.Join(",", parts);
        }


        return numero;

    }