Small scale brackish water desalination units are used in remote areas and their sustenance depend on the twin factors of consistency of product water quality and availability of raw water resources. A mathematical model has been developed based on the input parameters of feed salinity, basic membrane characteristics, feed temperature, desired product quality and quantity to predict the operating recycle ratio at constant operating parameters. This ensures the consistent quality of product water at the same time minimizing the raw water consumption. The algorithm used include calculation of recovery, determination of membrane configuration and operating pressure within the guidelines of manufacturer's specified hydrodynamic parameters. The model has a range with respect to reference value of design. The model can estimate the feed quality variation +10% to -70% and feed temperature variation +/- 5 degrees C based on the design reference value.