there was a bug yesterday (hotfixed now) where clients weren't getting a GCD on queued spells, effectively forcing *600ms*.
So if anyone played yesterday and thought "This 400ms window feels terrible", that's a misunderstanding. Give it another try.
It was actually "600ms, plus client not showing GCDs" that felt terrible.
My suggestion:
400ms is a good default
[ping+200]ms if you time/spam presses well.
[ping+100]ms if you're always a perfect god.
Anything less than 100ms is just wasting performance, IMO.
Regardless, if you're not sure, 400ms (default) is fine (post-hotfix)