Diameter based tracking for a window milling system. A window cutting system includes a cutting assembly with multiple cutting faces for cutting the window; and a diverter for guiding displacement of the cutting assembly relative to the window, the diverter including multiple separate guide paths for the respective multiple cutting faces. A method of cutting a window includes the steps of: positioning a diverter in a tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly; and then contacting the guide paths with the cutting faces to thereby form the window. Another method of cutting a window includes the step of: positioning a diverter in the tubular string, the diverter including multiple separate guide paths for respective multiple cutting faces of a cutting assembly, with each of the guide paths providing a separate wear surface for the respective cutting face.