A method and a system for determining bit values in a non-volatile memory having a number of cells each for storing a bit value are disclosed. The method includes the steps of: a) providing a first test sensing voltage to the cells and calculating a cell count; b) providing another test sensing voltage to the cells and calculating a difference of the cell counts between this step and previous step; c) providing still another test sensing voltage and calculating another difference of the cell counts between this step and previous step; d) processing step c) for N times; e) calculating differential amounts of cell counts and assigning an index number to each group of cells; f) choosing a voltage as an updated sensing voltage.