Implementing compressibility checking for Xvid X264 and if possible other codecs

[OvejaNegra]

can anybody explain to me the mechanics behind a compressibility check for xvid and X264 (well maybe is applicable to other codecs like theora or mpeg2).

The info i found on the network is a little ambiguos and confusing.
For example some people say that is only create a file with selectrange on avisynht to get the 10 or 5% of the video and make a compression with the desired codec using constant quality settings.The final filesize gives an estimate about how far or near you are from your target file size. If its to much the difference, then change resolution or apply filters to get close to the desired size.

But AutoGk does something different, it create a file called frames.log. What is it?
And using selectrange on avisynth does not make the codec use lots of keyframes for the many scene changes? Is not that a little artificial?

I want to make a tool to make easy the process of comp checking for me thats why i ask.
If the question is not appropiated for this forum section, please, feel free to move it to another thread.

Thanks again and sorry for my english