I went with ~1b per character precisely because OP stated that they use a passphrase. A long phrase with semantic meaning will be easier to remember than a 50-c

I went with ~1b per character precisely because OP stated that they use a passphrase. A long phrase with semantic meaning will be easier to remember than a 50-character diceware output, after all. Now, admittedly for me a passphrase is something that could be identified and processed by NLP-like construction. (Hence it could be also produced by one.) A sequence of random words requires, at least in my case, to come up with a special mnemonic. As a sibling post points out, if the phrase is generated by diceware (iow. sequence of words chosen at random from a large dictionary), then the entropy would be way, way higher - so high in fact that bruteforcing might be eclipsed by heat death of the universe.