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save> i think the problem can be summarized as this: given a number of frames between two keyframes, how many should be p and how many should be b frames? Ok, maybe im oversimplifying here because there could be a higher complexity in parts of this list of frames than in the others.
still, too many b frames will lead to unnecessarily large p-frames, and too few b frames will lead to too many p frames. the optimal number of b frames in the sequence is a number where if you add b frames the p frames will grow so that the total number of bits used in the sequence (given a fixed quantisizer) grows, and if you remove b frames the p frames added instead would also grow the total number of bits used. so the optimal ratio between b frames and p frames is one which yields the smallest file when using quality-based/fixed quantisizer encoding.
now, this is only a description of the problem and not the solution, since you do not want to do several encodes of the same frames just to see what number of b frames is optimal. there should be some way to determine this by looking at some factor, for example the same as used for determining the bitbudget and quantisizer settings.
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