Upon a read error, a flash memory controller adjusts a candidate reference voltage on successive read retries until either a read error no longer occurs or an optimal reference voltage is attained. Optimal reference voltages correspond to cross-points of flash memory cell voltage distributions. Cross-points can be determined by using different candidate reference voltages to read data from the memory and determining corresponding decision patterns. The frequency of occurrence of the decision patterns in the data read from the memory is used to conceptually construct a histogram. The histogram is used to estimate when a candidate reference voltage has been adjusted to a cross-point.