I has write about 237Gb on to tape by GNU tar and get those tapeinfo output:

# tapeinfo -f /dev/nst0
Product Type: Tape Drive
Vendor ID: 'HP      '
Product ID: 'Ultrium 3-SCSI  '
Revision: 'C26D'
Attached Changer API: No
SerialNumber: '**********'
MinBlock: 1
MaxBlock: 16777215
SCSI ID: 16
SCSI LUN: 0
Ready: yes
BufferedMode: yes
Medium Type: Not Loaded
Density Code: 0x44
BlockSize: 0
DataCompEnabled: yes
DataCompCapable: yes
DataDeCompEnabled: yes
CompType: 0x1
DeCompType: 0x1
Block Position: 1933551
Partition 0 Remaining Kbytes: 400308
Partition 0 Size in Kbytes: 400308
ActivePartition: 0
EarlyWarningSize: 0
NumPartitions: 0
MaxPartitions: 0

Firstly, Partition 0 Remaining Kbytes tell me that I no used any tape space.
Secondly, if I try to calculate the used space using the MaxBlock and Block
Position data, I get obviously wrong value: about 11.5% used. But my backup
data is not well compressible - it is video and photo. mt tell give me the same
thing: At block 1933551. How to estimate the used space on tape?