The FluksoUSB Board doesn't give any values below about 50Watts.
Because of the higher operating voltage than in the original flukso, the ADC-unit has an offset. To fix the gap you have to apply a little voltage as precharge. This is done by a simple voltage divider soldered to one terminal of input 4 and two pins at JP3.
A 20-round potentiometer (962-20 10K from Reichelt) has been used.
Connect pin 1 of a 10k potentiometer to pin 10(GND) of JP3 (foremost away from screw terminal), from pin 2 of the potentiometer to INPUT4 ( 2nd screw terminal from the left, if looked at from behind with screw terminals on bottom edge), and finally pin 3 to pin 1(VCC) on JP3. See also the following image:
Turn the potentiometer to the left. Now connect a single clamp between terminal 1(red wire) and terminal 9(black wire on sensor, INPUT4 on board) or 3 clamps in series between screw terminal 3 and 9(INPUT4). Turn the potentiometer to the right, until the value measured is bigger than 0W. Now slowly turn it back, until the measured power is 0W again. Now fluksousb is calibrated.