Since there is no way to know what blocking might be used in the future nor what number and sizes of objects might be placed on the tape nor what compression/compaction might be possible for the distribution of bytes that might appear in whatever data would be written, it’s unlikely that any “estimate” might be particularly useful. Lots of small objects should tend to take more space than a few large objects for example. And basic text might take less space than binary data (or not, depending on the bit patterns.)
I’m not sure if there’s even a useful way to determine how much physical tape is left. This has been the way tape has always worked, but current tape systems might be more predictable. I haven’t run across examples though.
Be aware that knowing how many bytes have already been written will not tell you how many more bytes can be written. It might get you close to the maximum possible to write under ideal conditions. The actual amount available will be somewhere between zero and the ideal maximum possible.