I think there is no easy way to do it. I would suggest you to generate a tape print with the command
DSPTAP DEV(TAPXX) DATA(*SAVRST) OUTPUT(*PRINT)
and then analyse the results adding the file sizes and do the math. You can also write a CL to do it.
It certainly is not easy to do and it is even so that any results that you may get are not exact. For once there is the tape capacity that is only an approxiate number. It could be that on one LTO2 tape you can write more bytes than on another.
Second factor is the compression rate at which your data can be compressed. This depends also on the nature of the data.
And than there is the wear and tear of the media. This could reduce the capacity of your cartridges over time.
I made the equasion derived from BRMS volume statistics with some LTO2 tapes (400GB capacity, upto 800Gb with compression) and found that 650 to 700 Gb were written on the volumes.
So you can determine exactly what is written on your tape, but what is the capacity of the Tape ???
Thanks all for the answer, through the SST my colleague here advised to see the write % of the tapes, that was quite a help to see how much it writes more recently and how many times as well. Thanks MCL and Peter…great help