If you have others, keep in mind their values stack (hence the loop pushing all the other values to 1.0, then setting the target one to the desired amount).
If your system doesn't have an /sys/class/backlight/acpi_video0, there should be at least one directory in there, which may be device-specific (I also have a radeon_bl0, for example).
Run it as root, with any value between 0 and 1.
# to an integer, even if $1 is a float (which it # The `/1` at the end forced bc to cast the result