1. A method of controlling a solenoid-operated fluid valve that has an output hydraulic pressure that varies in accordance with an input solenoid control current delivered from a drive circuit, comprising the steps of: providing a model characterizing the plant dynamics of the solenoid-operated fluid valve and the drive circuit, the model being dependent on at least one operating parameter that may vary; measuring the value of the operating parameter; adjusting one or more gain constants for a selected control strategy, based on the measured operating parameter value, using the model; and determining a required solenoid control current based on the selected control strategy including the adjusted gain constants therefor; and applying the required solenoid control current to the solenoid-operated fluid valve.