Trying to calculate a numeric value for the entropy of a password only makes sense if you understand the actual mechanism that is used to generate it. For example, it's relatively easy to work out the entropy for a password that's created from 3 random words out a list of 20,000, or that's created by choosing 8 random alphanumeric characters.
But you can't really work out a number based on "brain generated", because that covers a huge variety of different possible approaches. And even if you know the approach they use (such as picking a random word, and then sticking a digit on the end), humans can't do "random".
There also an unknown factor that people create passwords differently in different contexts. Most people will try and create (what they think is) a more secure password for their online banking than for some random forum they don't care about. So it's not enough to know how an individual creates passwords; you need to know how they created this password.
You could potentially try and calculate some kind of number based on how hard is was for you to crack that password - but that number would depends largely on how you were trying to crack it, so will be very variable.
TBH, any kind numeric value for the strength of a password should be taken with a big pinch of salt, and even more so if it's not a password that was computer generated.
So I think you'll struggle to calculate any kind of meaningful value - which leads to the question of what you're trying to achieve by doing so?