Hierarchical B pictures are devised to achieve temporal scalability in the scalable extension of H.264/AVC (SVC) which is under standardization. The fine-granular SNR scalability (FGS) can be provided by progressive refinement (PR) slices in SVC. In this paper, we firstly investigate error propagation in the case of discarding PR slices and obtain a rate difference distortion optimization criterion to improve coding efficiency of base layer. Then we consider the full rate case and propose a rate distortion slope criterion to enhance FGS coding efficiency at high rate. Finally the criterion to boost coding efficiency in the whole range of FGS rate is derived by combing the criterions derived previously. The proposed method is compared to the approach in SVC test model and up to 0.3dB coding gains are achieved.