The most favorable range for the YP/PV ratio of clay-free, KCl-polymer based drilling fluid systems will depend on the specific application and conditions of the drilling operation. Typically, a YP/PV ratio of around 0.5 is considered to be optimal for drilling operations, as it indicates that the fluid has a good balance of viscosity and gel strength. However, specific ranges may vary depending on the type of polymer used, the concentration of KCl, and other factors.