The problem is there is no truly meaningful answer to the question as posted for tape drives that do data compression, or for "virtual" tape. It is not widely known, but tape data compression, aside from possibly compressing the data as it is sent to the drive effectively wipes out your DCB parameters. The actual data is written in very long physical blocks, so there are fewer interblock gaps, a major contributor to tape drive usage, and especially with modern "streaming" tape drives, poor I/O performance. Another issue is a lot of tape usage these days is virtual tape. Several years ago my knuckles got rapped when I was trying to use real tape to measure real tape performance, rather than the "approved" virtual tape.
Still, knowing the number of blocks and their size can give you a rough estimate of the amount of data on the tape.