The question is purposefully ambiguous: I'm interested in various operationalization of "slack" as a fraction of one's resources. I'm interested in knowing the distribution across the present human population.
It could also be that the concept is fundamentally confusing in some ways, like we all have 0 slack because any action could make the difference between reaching longevity escape velocity and not. I'm also interested in comments along those lines.
For a post like this, I think you'll get much better answers if you offer a definition of what you mean by "slack", perhaps with some links to posts that originally coined the definition you are using (I assume you're referring in part to this post).
Thanks for the comment and the link! I don't remember this article. I generally agree with you about defining concepts, but here I wasn't even sure about how to formalize it