That's a bit redundant with current top post, but I think this site is really cool, not because of the calculator.
I think the calculator itself isn't really useful, though I could be wrong. (The reason why I think it's not really useful is that I think the "90 minutes" cycles varies quite a lot between people, and the time between going to bed and to sleep can vary by half that length between people)
But what I really like, is that it is kinda (reasonably) click-baity, while it actually is a short, sourced, clean, no-ad, straight to the point, FAQ about sleep, which is useful to raise awareness of those issues.