We have the resolution at 14-bit. The system returns the result (NRF_SAADC->RESULT.PRT). Because we return the value in millivolt we multiply that by 3600 and divide that by 2^14.
return (result*3600)>>14;
To get the value back you just have to undo this like the code below:
result = (millivolt<<14)/3600;