Flex duct is almost always turbulent airflow, and the Kele probe needs laminar (smooth) air flow to work.
It can be used with any manometer with in the range of the device, how ever a device that has higher precision and more resolution, like the DG 700 or DG 1000, yields better data than a testo 510 for example.
On a typical measurement of 100 CFM
the DG 700 accuracy with the Kele probe can be as close as +/-6 CFM, the testo 510 accuracy with the Kele probe can be as close as +/-21 CFM due to the accuracy of the pressure sensor in the DG vs 510.
You can input some data into the 3 open fields in this spreadsheet: https://docs.google.com/spreadsheets/d/1BVnl6OadDmVilmv-0cbaGZJai-cxFHZJcEhYwLgKHzQ/edit#gid=56598021