A system includes a pump, accumulator, a sensor which measures line pressure in a fluid circuit, and a controller. The controller plots and calculates respective slopes of first and second sets of measured pressure values from the sensor, calculates a slope ratio, and compares the slope ratio to a threshold. The controller also records the pre-charge pressure as the point of intersection of lines representing the slopes when the ratio exceeds the threshold. A control action is executed when the pre-charge pressure drops below a calibrated minimum threshold. A method includes measuring the pressure values, calculating the respective slopes and the slope ratio, comparing the slope ratio to a ratio threshold, recording the point of intersection of lines representing the slopes as an interpolated pre-charge pressure value when the ratio exceeds the threshold, and executing the control action.