Anyway, if you have the first 11 words and you want to have valid BIP39 seed phrase, there are 8 words that can be used as the 12th word.
There are (on average) 256 words which be a valid checksum for a 12 word seed phrase. It is 8 words (on average) for 24 word seed phrases.
Specifically what I'm trying to do is print out a list of the 2048 bip39 words and randomly select 12 to create my own offline generated seed.
Don't do this! It is an incredibly insecure method of generating a seed phrase. You will not and can not choose words randomly, despite your best efforts. Humans are not random. Whatever seed phrase you end up with at the end of this process will
not represent 128 bits of entropy.
I'm trying to ensure true ravdsomness in seed creation and this seems to be the only way I can come up with outside of being able to independently verify the code from wallet manufacturers etc.
Do not select words. Instead, flip a fair coin 128 times to create your entropy, calculate and append the checksum, and then encode that number in to the corresponding words.