Just hash the private key and use the as the next
Discussion
that means use 'sha256sum' btw https://linux.die.net/man/1/sha256sum
you have several options for the base too, you could derive off the npub, or the hex, or use xxd to turn the hex into binary, and then pipe that into xxd https://www.linuxjournal.com/content/doing-reverse-hex-dump