Data Compression on 4mm tapes

I had a questions asked of me today and really could not answer it well
so figured I'd see if anyone else knew.

Is there a way to calculate how much space a file system will take when
it is written to 4mm tape that is using data compression on the tape
drive?

Example:

Filesystem 1024-blocks Used Free %Used Mounted on
/dev/qclv 15728640 6071796 9656844 39% /qc
/dev/oldlv 15728640 11156748 4571892 71% /old
/dev/eomlv 15728640 10150848 5577792 65% /eom

I have a 4mm 4GB tape drive that offers 20gb/40gb when compressed. Is
there any way to calculate how much space will be left on this tape
after data compression in case we want to add another set of 18gb.

Uncompressed, I would need 45gb of space, but compressed it might just
fit.

Command used on this system is "cpio -ovC512"

Tape drive settings:

BLOCK size (0=variable length) [1024]
Use DEVICE BUFFERS during writes yes
RETURN error on tape change or reset no
RETENSION on tape change or reset no
DENSITY setting #1 38
DENSITY setting #2 37
Use EXTENDED file marks yes
Use data COMPRESSION yes

Thx In advance.

Steve

.

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