I'm not sure why is this an inconvenience, Centova Cast gives you the actual disk space utilization which is what you need to calculate how much space is left in your drive.
For example if you know that the size of the partition where the user files are stored is 1000GB, and Centova Cast reports 100 GB of 300 GB used, you know you have 900GB available, it doesn't matter if you allocate 300 GB or 2000 GB to your stations, Centova Cast will still report 100GB used, so you still know that there are only actually 900 GB left available on your drive.
For other scenarios, such as when you share the server with other services/panels, you could simply setup something like logwatch to send you a report of your disk space available once a day.
HTH