Convert and Parse Prefix Multipliers - C#

Introduction

The two functions convert/parse number strings with prefix multipliers (Milli, Kilo, Mega, Giga, etc.).

ParsePrefix

"124.67uH" >> 1.2467e-4
"124670uH" >> 1.2467e-7

"124.67mW" >> 1.2467e-1

"124.67MW" >> 1.2467e8

"100pF" >> 1e-10

AddPrefix

0.1123123 >> "112.3123m"

112312.3  >> "112.3123K"

1000 >> "1K"

1000000000000, "B" >> "1TB"

Here is the code:

        /// <summary>
        /// Parses a prefix number to double. Unit is ignored. 
        /// Prefixes greater than 1 ([K]ilo, [G]ega etc) etc are case insensitive. 
        /// (However, m is milli, M is Mega)
        /// </summary>
        /// <param name="value"></param>
        /// <returns></returns>
        public static double ParsePrefix(string value)
        {
            string[] superSuffix = new string[] { "K", "M", "G", "T", "P", "A", };
            string[] subSuffix = new string[] { "m", "u", "n", "p", "f", "a" };
            char sufChar = '\0';
            foreach (char c in value)
            {
                foreach (string s in subSuffix)
                    if (c.ToString() == s)
                    {
                        sufChar = c;
                        string num = value.Substring(0, value.IndexOf(sufChar));
                        return Convert.ToDouble(num) / Math.Pow(1000, 
                               subSuffix.ToList().IndexOf(sufChar.ToString()) + 1);
                    }
                foreach (string s in superSuffix)
                    if (c.ToString().ToLower() == s.ToLower())
                    {
                        sufChar = s[0];
                        string num = value.Substring(0, value.IndexOf(c));
                        double mult = Math.Pow(1000, superSuffix.ToList().IndexOf
                                      (sufChar.ToString()) + 1);
                        return Convert.ToDouble(num) * mult;
                    }
            }
            return Convert.ToDouble(value);
        }
        
        /// <summary>
        /// Selects a suitable prefix for the value to keep 1-3 significant figures 
        /// on the left of the decimal point.
        /// </summary>
        /// <param name="value"></param>
        /// <param name="unit">Optional unit to be put after the prefix</param>
        /// <returns></returns>
        public static string AddPrefix(double value, string unit = "")
        {
            string[] superSuffix = new string[] { "K", "M", "G", "T", "P", "A", };
            string[] subSuffix = new string[] { "m", "u", "n", "p", "f", "a" };
            double v = value;
            int exp = 0;
            while (v - Math.Floor(v) > 0)
            {
                if (exp >= 18)
                    break;
                exp += 3;
                v *= 1000;
                v = Math.Round(v, 12);
            }

            while (Math.Floor(v).ToString().Length > 3)
            {
                if (exp <= -18)
                    break;
                exp -= 3;
                v /= 1000;
                v = Math.Round(v, 12);
            }
            if (exp > 0)
                return v.ToString() + subSuffix[exp / 3 - 1] + unit;
            else if (exp < 0)
                return v.ToString() + superSuffix[-exp / 3 - 1] + unit;
            return v.ToString() + unit;
        }