I took the index with the words, transformed the number to binary, extra a few zeros, calculated the sha256 digest, took the first Element of the hexadecimal to transform it to binary, additional 3 zeros to your eight digits on the binary and got 00000010011 which translates to 19 in https://top10bookmark.com/story18824954/considerations-to-know-about-bip39-seed-phrase-generator