ncdu / du and df show different sizes: du is reading information from the directory tree, it is more accurate than df but slower.df reads meta data of the file system therefore it is faster, but not that accurate because it works with blocks...
Here is a small script that will send you an email when the disk usage rises above the percentage specified by the THRESHOLD variable (90% in the example bellow). If you don’t want to step up to a full monitoring solution such as Nagios...