I'm interested in this as well. My personal rule is that I'll spoil it if it's either extremely twist-y, like say in The Usual Suspects or Primer, or something that came out recently, like an ongoing TV show.
I'm pretty sure I just got a post edited for spoilering
the fact that the clone troopers in Star Wars turn out to be the stormtroopers in the old trilogy. But I don't think this was supposed to be a revelation - the clone troopers intentionally look almost exactly like the stormtroopers. I spoilered that part more as a "I'm embarrassed about this".
[broad Star Wars spoiler?]
I understand the need for spoilers - hell, I'm usually the one yelling at people for spoiling Game of Thrones, but I also think that if we have a spoiler tag, we don't need an additional spoiler warning and 10-15 lines. It breaks the flow of a post, and is imo redundant. While I understand that people might unthinkingly expand the spoiler tag and spoil things big time for themselves, what I do is just to put the scope of the spoiler in parentheses above or below the spoiler tag, like [book 4 spoiler], or [season 1 finale spoiler], which both makes it clear that it actually is a spoiler, and broadly what it spoils.
That said, I of course, don't mean to tell you mods how to do your job. Just a suggestion.