Is there a line/guideline for how much reality can be involved?
There are no official rules here — if whatever you're doing is legal, it's fine. However, most of the community doesn't take well to people exploiting real disasters (we get a lot of Chernobyl-related deletions, for instance).
For example, I have really creepy photographs of a cave some friends and I explored.
I would like to give some specific details I'd like in there, and then give it to some SCP writers to come up with a story.
Ruleswise, this is totally fine. However, don't be surprised if 'SCP writers' don't really want to write about your cave — most of us have ideas of our own we want to pursue.
You can feel totally free to make a thread somewhere (Ideas and Brainstorming is generally good) with the details you think would make a cool SCP and the photos and see if anyone is interested, though.
Or say I wanted to explain Benedict Cumberbatch's fame and our ability to recognize any form of his name no matter how ridiculous by making him an SCP.
This is also fine. I mean, we have a John Cena article on the site. However, I'd doubt people would really like this without an interesting, novel approach to it. Benedict Cumberbatch jokes are already considered kind of overdone and unfunny.
