Text, Context, Subtext

I stayed up far to late last night, and spent a little bit of time re-watching old Vlogbrothers videos, and this one got me thinking:

If you didn't watch it, the key quote to my point (which I quote here unironically) is:  "It's impossible to pull a line or a sentence or even a chapter from a book and understand the meaning of that section. Because as much as it pains us in this soundbitey twittery world, text means nothing without its context."

Now, I generally agree with that claim.  I don't think you can really understand the meaning of any given line from a book or poem unless you've read that book or poem, and know at least a little bit about the context within which it was written.  Of course, recontextualizing it can allow it to take on new meanings, that the original author might never have intended.  But in a larger work, authors can work pretty hard to develop a sense of context and subtext that inform the interpretation of every subsequent sentence.  For example:

"Oh yes. Richard and Anthem 2.00. Susan, that thing has got to be in beta testing in two weeks.  He tells me it's fine.  But every time I see him he's got a picture of a sofa spinning on his computer screen.

That's from the book I'm currently reading, Dirk Gently's Holistic Detective Agency, and won't make proper sense if you haven't read much of the preceding page and at least one chapter that turns up later in the book.  In fact, that sentence might turn out to have significance I can't even begin to appreciate now, before I've finished reading the entire novel.

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So, obviously we can't take a single sentence and assume we've got the whole context.  But on the alternative end of things, do you have to understand everything, know everything, have an intimate appreciation for the whole of the history of the universe, before you can claim to appreciate anything within its proper context?

That belief cracks under even the slightest pressure of pragmatism -- if there has to be a level of context we can deem 'close enough to live by,' that's certainly not it, because you can't live by it.

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My question, then, is:  is there any way to design a sort of criteria test for figuring out how much context one should provide?  Does such a test or standard already exist?  Ideally, it would be clear enough that anyone with a reasonable interest in whether they're miscontextualizing would be able to apply it, and transparent enough that in cases of obvious exceptions, one should be able to explain why the rules are different in that case.