syntax is {number}{optional %}{optional +-}
So:
5%+
adds 5% of max brightness to current brightness.50
sets absolute value of 50.50+
adds 50 to current brightness.5%
sets to 5% of max brightness.
Usage: brightctl [OPTIONS] [COMMAND]
Commands:
i / info
g / get
m / max
s / set
help Print this message or the help of the given subcommand(s)
Options:
-l, --list
--pretend
-m, --machine-readable
-n, --min-value <MIN_VALUE> N{%}{+-} [default: 1]
-d, --device <DEVICE>
-c, --class <CLASS>
-h, --help Print help
-V, --version Print version
Some udev rules are provided to be able to use this cli tool.