Grove GSR sensor calibration

Hi guys, I'm using the GSR grove sensor (seed) for a project. Unfortunately i was not able to find a complete and useful datasheet (seed website is giving very limited information).
In their website ( they suggest to calibrate the sensor at 512 and then use ((1024+2*Serial_Port_Reading)*10000)/(512-Serial_Port_Reading) to compute the final resistance.
I can't calibrate the sensor at 512 and i bought a new one thinking that the problem was in my sensor but is still does not work.
i'm able to change the potentiometer until 760 and then it goes sharply to 0.
however I was thiking on leaving the calibration at 760, but then i don't know how to compute from the adc output the corrispondent resistance.
do you have any idea on how to change the previous equation? i tried to resolve the schematic but I wasn't able to find any input/output relation.