I don't know if you know this already, but it's also about filesize prediction and thus may prove useful for you:
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The kvcd formula is supposed to work with a gop-length of 15, but that's not a problem I guess. Alas, I can't find a link for the "Sampler" avs-plugin, but I have it on my hdd (please pm me if you need it). I used this method myself several times til I got tired of reencoding the sample avs until the filesize was right.. nonetheless, quality IS great