A working machine such as a road milling machine of an asphalt paving machine has an automatic grade and slope control system that causes the machine to maintain a pre-selected grade and slope with respect to a reference without significant operator input. A calibration system employs a calibration controller to calibrate the automatic grade and slope control system. The calibration controller causes a current to be sent to a valve that operates a linear actuator that may be employed to change the attitude of the frame of the working machine. The calibration controller uses feedback from at least one sensor, such as a height-measuring sensor or a slope-measuring sensor, in an iterative process that determines the minimum current necessary to cause a change in the attitude of the frame of the working machine. Once this minimum value is determined, the system will establish the optimum settings for the automatic grade and slope control system to improve its efficiency and cause it to operate more smoothly.