Doesnt sound like your requirement is anything too out of the ordinary. I would run some metrics using the system log to see how much space you use. Export the 87 records into excel, and then come up with your daily use, then take a baseline. Times that by 30 for a month.
A common process I guess you could say would be to keep your documents in cache for X days, 10,15,30,60, etc.. For commonly used application groups for performance reasons. Then, after that threshold has been met..Use ARSMAINT to migrate that into TSM, which should be backed up nightly. Several installs I have worked on have had that kind of backup policy.
Also take into consideration your document types and compression when figuring out how much space you use. I am not sure what you are trying to accomplish here--Could you elaborate?
The first cache file system contains special directories used by Content Manager OnDemand to locate objects from all cache file systems, is it possible to create more than one arscache that fulfills the function of the primary?