In some embodiments, a gas distribution apparatus may include: a manifold having a gas inlet to receive a process gas from a fast gas exchange unit and a first gas outlet to provide the process gas to a first gas delivery zone; a plurality of flow restrictors fluidly coupled to one another in parallel and to the gas inlet, wherein each of the plurality of flow restrictors are configured to allow at least a portion of a total flow of a process gas through each of the plurality of flow restrictors; and a plurality of valves each coupled to respective ones of the plurality of flow restrictors, wherein the plurality of valves are configured to be selectively opened to allow the process gas to flow through selective ones of the plurality of flow restrictors to provide a desired percentage of a total flow of the process gas to the outlet.