I took the index with the words, converted the number to binary, extra a few zeros, calculated the sha256 digest, took the initial Element of the hexadecimal to transform it to binary, included three zeros to your 8 digits in the binary and obtained 00000010011 which translates to 19 in https://johsocial.com/story9417101/5-simple-techniques-for-bip39-seed-phrase