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Absolutely required here: xkcd.com/936 . Beware more of a joke than a serious anwser hence only a comment. But it shows that the length is more important than the number of character classes when you try to actually compute entropy.
The math involved is about entropy. We have several questions about that here, many of which reference the XKCD comic above. The practicalities involved include things like known passwords, reused passwords, etc. So, in short, there is no simple method to calculate. All "password strength calculators" involve quite a lot of assumptions. Which is why this was closed as opinion-based, because everyone has their own assumptions about that. However, my comment above about adding these specific strings to a password list cannot be dismissed.