That's not the formula and many RPGs don't show you detailed numbers like this. I'll consider adding it to a tooltip in the future when you hover over defense, but this is a more casual RPG. The numbers aren't meant to be obsessed over and I don't want to overwhelm the player with a bunch of numbers.
The formula is:
f = (defense/ 200)
f / sqrt(0.2 + f^2)
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I apologise for being so confidently wrong there. I squared the full sqrt function instead of f which led to results that I could simplify to the version I shared, and I didn't try testing this in-game to confirm. Now I want to know, how did you create/discover this formula and why did you choose it instead of a more common alternative? I don't think I've seen anything like this one before.
I created it. It's derived from a sigmoidal curve function. I dabbled in them when creating my own neural networks long ago.
Defense in the game will be soft-capped around 200 and ~90% damage reduction. I didn't want to hard cap damage reduction like The Elder Scrolls. If a player finds a way to squeeze out more defense then it will continually be effective but never reach 100%. The 0.2 controls how steep it climbs, making it more effective early on and when it drops off.
I don't know more common alternatives. I just used what I know. I doubt it's a good function, I tend to hack stuff together. It had the shape I wanted in Desmos.