@trekk
The file to play around with is in
frontend/device/kobo/sysfs_light.lua.
@JSWolf
I don't know what it looks like on the One, but on my H2O the only frontlight level I occasionally use is 1 and everything over 10 is absurdly bright. If there's more precise control to be had in the lower brightness levels, that's where it's at as far as I'm concerned. But only people with that kind of frontlight can properly play around with this stuff.