A method for determining a liquid level of a reservoir includes sensing a plurality of liquid level points during a plurality of defined computation intervals, and determining filtered block points from the sensed liquid level points occurring during each of the computation intervals. The method fits a regression model to the determined filtered block points, including determining a slope and a standard error of regression, and determines an actual liquid level change from the slope of the regression model. The method may also include making a diagnostic determination if the method determines that a confidence ratio is sufficiently high.