For words that only have three letters, there is absolutely no 4th letter. For instance with the word “add”, there won't be any a lot more letters Later on and therefore have to be the word "insert". The word “addict” is from the BIP39 wordlist however you would've utilised “addi” https://express-page.com/story4189599/getting-my-bip39-words-to-numbers-to-work