LTO tape capacity less than expected, how do I calculate?

[jb_wisemo]

We are using a HP(E) 1760 tape drive to do D2D2T backups via customized scripts.  However in recent weeks I have noticed that we get tape full errors ("No space left on device") after less than 680780011860 bytes written. Despite turning off hardware compression and the tapes being nominally 800000000000 bytes.

Is there are formula to calculate the exact number of bytes that can be written to an LTO tape, and what  factors (such as I/O block size or other soft conditions) affect the available capacity?

I know that LTO-4 drives are not the current generation, but LTO specifications require that we do a hard reinvestment in media and drives to do any switch to a different generation.  Thus when we last had a drive failure, we opted to purchase an identical replacement drive from the 2nd hand market, as HPE was quite unhelpful with repairs and parts availability.