Systems and methods are provided for determining optimal read reference voltages used for reading data in non-volatile storage devices. A method may include reading data stored in a non-volatile storage device using a group of soft read reference voltages, decoding the data and obtaining the number of ones and number of zeros for each of a plurality of zones delineated by the soft read reference voltages, determining that one of the soft read reference voltages is a boundary of a zone in which a comparison result of the number of ones compared to the number of zeros is greater than zero and a boundary of another zone in which a comparison result is less than zero and setting the soft read reference voltage adjusted by an adjustment as an optimal read reference voltage. The adjustment may be obtained based on the two comparison results.