Hi

Did you try below current calibration?  And for 300A scale, there may be larger error for small current. The meter scale should be closed to actual current.

Charging /discharging current calibration：

1. Hook it up according to the charging/ discharging wiring diagram.
2. Set the power supply as above 12V, then hold the OK button and power the meter simultaneously. Hold “OK” for 3 seconds and then release. It will come to parameter setting;
3. Please move yellow cursor to “Mode” and short press OK button, then move the yellow cursor to “+4.A”(It is “-4A” in discharging mode). In that case, please make sure the actual load current is over 1.00A. Next, please press up or down button to adjust the current reading of meter until it is the same as actual load current. After that, press OK to move yellow cursor to ”Save” and save the calibration.

At last, power off and then power on again.