Calculating Kincaid Readability Index

 class Readability
    {
        public double CalculateReadability(string Passage)
        {
            string[] sentences = Passage.Split(new[] { '.', '!', '?' }, StringSplitOptions.RemoveEmptyEntries);
            double avgSentenceLength = sentences.Average(x => x.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Length);
            double avgSyllablesPerWord;
            int syllables = 0;
            int words = 0;
            foreach (string sentence in sentences)
            {
                foreach(string word in sentence.Split(' '))
                {
                    syllables += CountSyllables(word);
                    words++;
                }
            }
            avgSyllablesPerWord = syllables / words;
            int numberofSentences = sentences.Length;
            return 206.835 - (1.015 * avgSentenceLength) - (84.6 * avgSyllablesPerWord);
        }


        private int CountSyllables(string word)
        {
            char[] vowels = { 'a', 'e', 'i', 'o', 'u', 'y' };
            string currentWord = word;
            int numVowels = 0;
            bool lastWasVowel = false;
            foreach (char wc in currentWord)
            {
                bool foundVowel = false;
                foreach (char v in vowels)
                {
                    //don't count diphthongs
                    if (v == wc && lastWasVowel)
                    {
                        foundVowel = true;
                        lastWasVowel = true;
                        break;
                    }
                    else if (v == wc && !lastWasVowel)
                    {
                        numVowels++;
                        foundVowel = true;
                        lastWasVowel = true;
                        break;
                    }
                }

                //if full cycle and no vowel found, set lastWasVowel to false;
                if (!foundVowel)
                    lastWasVowel = false;
            }
            //remove es, it's _usually? silent
            if (currentWord.Length > 2 &&
                currentWord.Substring(currentWord.Length - 2) == "es")
                numVowels--;
            // remove silent e
            else if (currentWord.Length > 1 &&
                currentWord.Substring(currentWord.Length - 1) == "e")
                numVowels--;

            return numVowels;
        }
    }