As far as fiction inspired by a true story (and supported by meticulous – woo-hoo- research), there are two good reasons to call it “Fiction” instead of non-fiction.

A. You may need to prove your expertise on your subject to get a publisher to take notice, and in most cases, you will lack the expertise.
B. Most important reason to call it fiction: to not get sued – to not get sued and to not . . . well, you get the idea. However, in regard to handing over all your earthly goods to an undeserving plaintiff with tons of money and clout who sees himself/herself in your cleverly disquised “fiction,” a recent court decision found for the writer. In this case, even though the plaintiff could clearly show that the writer was describing him when she told the story of having been physically abused, the district court declared that writing that contributes to the common good is protected by the first amendment.
Note: Most home owners policies have a rider available (for an additional, nominal charge) that covers the writer for up to twenty million dollars in case of a libel (or slander) suit. If you are writing about an organization or person with the ability to hire many very expensive lawyers – even though you can prove what you are saying is true – you will still have to spend megabucks to defend yourself, making the rider a very good investment.

If you say it’s fiction, then presumably you’re working your meticulous (I love that word — it always sounds so good in my head) research into the framework of the story you want to tell. There’s always the issue of how the research is or isn’t used. I often use the example of medieval stories — people can research the era throughout their lives, but they cannot truly know what it was like to live back then. Both fiction and non-fiction authors make assumptions based on what they know and what they believe. If you say it’s non-fiction (and, especially, if you say it’s a true recounting — caveats noted about — of your life), you will be held to a higher standard of credibility.

There is incredible blurring of fiction and non-fiction, and, as you noted, rather than strict breakdowns between them, it’s a continuum — the author, primarily, chooses where the work falls. The reader may beg to differ or accept the author’s designation of the work.