on my phone, i let it adjust automatically, but always have it on 90+ percent reduce white point so it's even darker than normal. on my computer it's usually 2-3 notches (mac). on my DS it's automatic, but around 3 at default and i change it if i need to.
I always have it as low as it can get on my computer and 3ds's, and even that feels like too much for my eyes sometimes. lol I only turn it up if I have my 3ds outside because I can't see the screen otherwise.
Most of the time I have it on pretty low unless I'm out in the sun then maybe it's halfway to 70%? At night when I'm using my phone in my room I have it even lower than the lowest lmao... There's like this trick on iOS that can lower your brightness beyond what Apple gives you.
On my phone and labtop it's slightly higher than the lowest setting but not super high, higher on my labtop than my phone, but on my 3ds I think it's on 2? I can't really remember exactly