It's hard to calculate how much free space is left on a tape. Amount of data you can store on a tape depends on compression you are getting on the backup data. You can run available_media, which is in goodies directory to get an idea on how much data is stored on each tape.
I have a VBScript for Windows that could be tweaked to produce a listing/CSV of actual amount of data stored per media.
Across 1250 media, I do get an average of 200GB per LTO1 media. I get 215 GB average in one library, and 185 GB in the other library - one houses mostly production clients, and the other mostly UAT/test/dev clients.
Don't forget that retention also determines overall how mauch data can go on a tape because Netbackup does not mix retentions. So if you have an 18 month retention for 1 policy then onlly the data in that particular policy will go on the tape.
How netbackup manage with expiry date of the media and images on the media ? i.e will the media expire before the images on particular media expire ? it should not happen like that so can any on tell the behaviour of media and its images ?