The values returned are the raw integers read from the ADC (analog to digital converter) on the output of the transimpedance amplifier (current to voltage converter). You will need to convert these integer values to current measurements e.g. uA. While it is possible to do a first principles calculation using the values of the feedback resistor in the transimpedance amplifier it is generally better to just do a simple calibration using a dummy cell consisting of a single resistor (of known value R).

The basic idea is to connect the counter and reference electrodes to one side of the resistor and the working electrode to the other side. You then want to sweep the voltages through a range of values - e.g. via a cyclic voltammetry test. Because the resistance R is known for each output voltage v(t) in the test you can calculate the current i(t) = v(t)/R through the resistor. At the same time you have the measured ADC integer values n(t) corresponding to this current. To get the calibration you can just perform a linear fit between the ADC integers n(t) and the know current values i(t).

When performing the calibration test make sure to select an appropriate resistor. You want to generate currents which span the input range (for current), but don't go outside of it. The transimpedance amplifier can't generate voltages higher then 3.3V or lower than 0V - so if you go outside of the current range you will get clipping (or saturation) of the output which will show up as a flat spot in your calibration data.