I'm trying to set a limit for maximum screen brightness on my Thinkpad X250 running Ubuntu 18.04.
A bit of context : I changed the original screen with a new one (before installing Ubuntu), the brightness controls work perfectly, but I noticed that, over 55-60% brightness, changes are almost unperceivable for human eye, but it drains a lot more battery and it starts running really hot. I'm planning to give the computer to my family with young and unexperienced users, so I would like to set a limit which does not require user action at each startup, and could work for all users without worrying that an update via the software manager could erase the settings.
What I tried : I tried to directly edit /sys/class/backlight/intel_backlight/max_brightness but it refuses to be edited (even after using chmod to change permissions). I saw some anwsers telling to add a command like "echo MAX_VALUE > /sys/class/backlight/intel_backlight/max_brightness" in /etc/rc.local, but I don't have this file. Note that I don't have *acpi_** files or folder in /sys/class/backlight/.
What would be the correct way to set an all-user permanent brightness limit ?