When changing the screen brightness using FN keys, there are only 10 increments in the brightness settings. The lowest one is a black screen and the next one is already mid-bright for me. So basically it is not possible to set a low screen brightness level.

The following lines in common/xfpm-brightness.c seem to be the cause of the course brightness steps.

  brightness->priv->step =  max <= 20 ? 1 : max / 10;
  brightness->priv->step =  ret <= 20 ? 1 : ret / 10;

I wonder if you could change these numbers to 20 or 30 to allow finer steps, e.g.

  brightness->priv->step =  max <= 30 ? 1 : max / 30;

I saw there is something called xfce4-settings-editor. Perhaps you could put the number of steps as a parameter in there so users can do whatever they want?