I’d agree with Ben, all you can really do is measure the ‘tape hours free’ and say what % busy your overall environment is.  You can I guess also see the total data moved in that backup window or 24 hour period, multiply the ‘% free’ by that and get an approximation at how much more data can be backed up with what you have.

Now of course it only really works if your environment is totally homogenous, for example there is only one storage unit/storage unit group or SLP for all backups, and underlying it is one media server or a pool of media servers.  All backups must be over the same speed of LAN….not many backup shops are like that.

Otherwise you will calculate you have free capacity, but actually it is all on one SAN media server that does not 100% use its tape drives, and the users (curse them!) will add the extra data somewhere quite else.

If you can move your architecture towards being able to load balance ‘everything’ it will make your life easier.  Reporting tools like EMC DPA, OpsCenter Analytics etc can help you figure out where the capacity is and where the slowdowns are that can be addressed to sweat the assets more.