Hi
The du commands posted should be adequate for your requirements. However, always remember that the percentage is relative ie. if you have 100 gigs of which 98% percent is used, you still have 2 gigs available. What you should look for are files (eg log files) that grow on a regular basis. Once you have established which files these are, implement a procedure whereby these files are regularly archived and then purged. Its not about how much space you have but rather how you manage it.
Cheers