How to calculate an accurate estimated reading time of text?

Question

I suppose the calculation should not be done by only two factors (average reading speed/words per minute, and word count). But at least by a third parameter, that in my opinion should measure the difficulty of the used vocabulary with some kind of text analysis.

I suppose it would be something like this:

totalWordCount / (avgReadTimePerWord * textDifficulty)

Also I read somewhere that the average reading speed is 120 words per minute. But I have some questions regarding that:

1. Are people known wich existing algorithms that are capable of doing such accurate calculations considering those factors?
2. Is the average reading time equal in all languages and if not, how to add language as a factor also? What study shows average reading times per language?
3. Is it possible to add age as a factor? Or are there known studies on those factors?
4. If someone has dyslexia. Is their some research that suggest for example they take 30% longer to read? And can we just increase it by 30% in such case or are there other factors in such case?

Goal is to implement this as a function in a programming language like Python or PHP and to use the (personalised) result for creating an output like "this article can be read in approximately 12-14 minutes".

It would be nice if such information is publicly available.