Embodiments of the present invention generally relate to methods of controlling gas flow in etching chambers. The methods generally include splitting a single process gas supply source into multiple inputs of separate process chambers, such that each chamber processes substrates under uniform processing conditions. The method generally includes using a mass flow controller as a reference for calibrating a flow ratio controller. A span correction factor may be determined to account for the difference between the actual flow and the measured flow through the flow ratio controller. The span correction factors may be used to determine corrected set points for each channel of the flow controller using equations provided herein. Furthermore, the set points of the flow ratio controller may be made gas-independent using additional equations provided herein.