Abstract: A method for controlling a system parameter based on controlling a setpoint for a process control variable includes measuring the rate of input to and output from a vessel. The difference between the input and output rates is determined to provide an expected rate of accumulation in the vessel. The actual rate of accumulation in the vessel is measured. The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term. A value for the setpoint for the process control variable is calculated by solving one of a material or energy balance equation which includes the bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. The setpoint for the process control variable is then controlled based on the calculated setpoint value. An apparatus is disclosed for carrying out the disclosed method.