There are a couple of this to watch out for with estimated capacities.
They use either default or user-defined values as a baseline.
They can also be offset by compression on the drive, or in software (either
ADSM itself, or client-compression, e.g. file compaction in NT).

You also need to watch out for the fact that due to it's incremental nature,
ASDM can often remove one verion of a file from a tape's inventory because
it's too old, or has been superceded by a newer version of that file
(usually on a different tape). This, over a period of time, causes tapes to
become less and less utilised, but they remain at a status of "FULL".

Part of the DEFINE DEVCLASS command lest you specify an ESTCAPACITY.
You could Query DEVclass to see how your system is currently set up.

        Hope this helps....
                Craig McAllister, Tech Support Monkey