Gonna weigh in here, and play a bit of Devil's Advocate.
There are a SHIT-TON of writing guides full of thought-exercises and writing prompts. I've got a couple, myself. They are
all, essentially, pairs of variously tinted lenses through which to view and analyze your works: Some people say it's all about being spontaneous and connecting with your subconscious mind: Others stress various plot-structures, like character arcs and a four-act structure: Personally, I think Stephen King's
On Writing is "the best" single book about that stuff, but it's more of a "writing 101."
Anyway, I suspect that's the original spirit, of this 50/50 rule. There's a SHIT TON of those too- little writer's rules of thumb. Here's a couple, off the top of my head.
1) Never write in 2nd person.
2) Adjectives are bad.
3) Show, don't tell.
4) Always be sure to include information about all five senses.
5) Write what you know.
That's off the top of my head.
Obviously, there are
successful exceptions to
each of these rules. But, I'd like to point out, that a couple of outliers, don't negate the essential wisdom of the doctrine. For the most part,
you're better off writing what you know. Adjectives
should be avoided, if you're describing physical actions. Your writing will be stronger, as a result.
It's a good idea,
in general, to try and include women characters. Certainly, I've written stuff before, and gotten feedback that was "I liked it, but there weren't any women characters." And then I was like, "Shit, I totally didn't even think about that." A lot of times, it's not a whole lot of work, to make a character female, after you've initially written the character as male.
So anyway, I suspect that's the origin of this 50/50 rule, and some SJWs got a hold of it and went crazy.
PS-
Here's another of those writing books that stands out in my mind. I
suspect Judd Apatow has read it. It's
nominally about screenwriting, but storytelling is storytelling,
non?