The Medium-Granular scalable (MGS) technologies in H.264/AVC-based scalable video coding (SVC) provide a flexible foundation to accommodate different network capacities. In order to make use of MGS in multiple environments conveniently, we need to obtain the Rate Distortion (R-D) function of SVC and design efficient bitstream extractions. In this paper, we proposed a simple and effective distortion model to estimate the reconstruction distortion with drift error. Based on the model, a simple and fast weight-based priority setting algorithm is designed to achieve optimal R-D performance in MGS bitstream extractions. A smooth extraction algorithm is also provided for smooth quality substream extraction. Extensive experiments show the accuracy of the new R-D models and the effectiveness of the proposed extraction algorithms.