The thing is for your personal bank account a 15 character password is acceptable.

But for x many customer credit card details you're really looking for a much longer password that that. I'm talking 64 characters or more of pure random data.

You shouldn't be compromising for the convenience of being able to remember a password when it secures such critical data in my opinion.

Edit: I do agree though that your method is a very good way of remembering password.