A gas distribution plate assembly and a method for distributing gas in a processing chamber are provided. In one embodiment, a gas distribution plate assembly includes a tuning plate coupled to a diffuser plate. The tuning plate has a plurality of orifice holes formed therethrough that align with a plurality of apertures formed through the diffuser plate, where the apertures each have a greater sectional area than the holes in the tuning plate. Each aperture is aligned with a respective hole to define gas passages through the gas distribution plate assembly. The tuning plate may be interchanged with a replacement tuning plate to change the gas flow characteristics through the gas distribution plate assembly.