I am not a cryptographer, but I'll offer some advice, if I may: don't let people choose a passphrase, generate one for them. People are very bad at creating good passphrases, but decent at memorizing a good one.

I say that because I don't trust the zxcvbn library. It underestimates the entropy of "aaaaa" as 7 bits [log(26 * 5)], not the correct value of 23 bits [log(26) * 5], for example. In this instance, it's to your advantage, but it doesn't inspire confidence in its other calculations.