Collecting allocated space and used space for sequential and "regular" partitioned data sets from the VTOC is not difficult. You are most of the way there using IEHLIST LISTVTOC FORMAT, though, as you have already determined, interpreting the output is not so easy.
As both NicC and Mr. Scherrer have mentioned, this analysis leaves many missing elements.
- Data bases
- Data sets migrated by HSM or ISV provided utilitiies (CA-Disk, for example) that perform similar functions.
- PDS-E data sets
- Data sets created during job execution.
- Tape data sets
Even when allocated and used space is available, determining if there is a potential problem can be difficult.