When you should stop read a book?

I like to read, and from more than year I try to spend daily at least 45 minutes reading some popular science or similar book.

But sometimes I don't like the book.

And I'm looking for some "formula" to decide if I should stop to read or not.

I already spend this time, and I'm not able to recoup it, still I may try to rescue rest of time I would spent reading....

So maybe something like:

expected_value = value_till_now/time_spent

This will let me estimate how much I may gain.
Interesting fact, because at the beginning I have some "value" already at least from this that reading on its own improves vocabulary and so on, at the beginning expected_value is almost infinite ;-)

Now is problem of this what should be expected_value.
I would propose something radical, expected value should be at least equal to average value from previously read books.
Let even be nicer for book, lets say it should be at least half of value of average book.

So formula to make decision should be:

stop_reading = value_till_now/time_spent < 0.5*average_value_of_book

It seems also that using this formula will cause that reading more books will means stoping earlier with bad books ;-)

And thanks to this I avoided in last year or so reading books like:
The Singularity Is Near by Ray Kurzweil - topic was interesting, but at least Polish translation of this book was so badly written that I didn't wanted to read it,
Getting Things Done by David Allen - how many times the same may be written? Or maybe this was again Polish translation,
The 4-Hour Workweek by Tim Ferriss - I admire Tim Ferriss for creating really good idea of making business - let others do it - it brillant idea, and books created in such way like Tools of Titans, and Mentors Tribe are quiet interesting, this one is simply boring.
Capital in the Twenty-First Century by Thomas Piketty - main idea of book is worth noting, but it isn't easy read, and one simply don't have way to verify data provided, so instead of reading this one book I would prefer reading some summary of it (from this what I heard the same is true about Marx's Capital ;-))

Similar posts^beta
Python yet another hero of SF book Seveneves
How to detect clickbaits?
Recursion is evil ;-)
It's alive... still ;-)
How to get negative number from size() in LinkedList in Java? ;-)