A method of and a multiple zone pressure controller system for controlling the pressure of a gas or vapor flowing to at least two zones of a process tool such as a vacuum deposition chamber. The system comprises: at least two channels configured and arranged so as to provide the flow of the gas or vapor to corresponding zones of the process tool, each channel including a pressure controller configured and arranged to control the pressure of gas or vapor in each channel, a leakby orifice or nozzle configured to provide a leak rate of gas or vapor from the channel; and a controller configured and arrange to determine the true flow information to each zone of the process tool so that the true leak rate in the chamber can be determined.