Looking at my Flexbackup log it says:
Total Bytes written: 604805120 (577MB, 1.2MB/s)
Kilobytes Out 238340
It would seem that Flexbackup is compressing my mixture of word processing, spreadsheet, database and JPG's by about 2.5x. If it was mainly JPG's that would be considerably lower.
BTW Dat72 drives are now about the same price as DDS4 so why not go up a generation?
HTH
Neil