The stuff described in your link is pretty worthless. See here why:
http://forum.doom9.org/showthread.ph...511#post841511
http://forum.doom9.org/showthread.ph...12#post1311212
I'm not 100% sure what the going method on compressibility tests is, but you could:
1) Find a CRF value for which you like the quality
As a start look at the range of 20 - 24. Higher number is lower quality.
2) Encode x% of your movie with that CRF value
You can use the avisynth function SelectRangeEvery(2000, 100) for instance to encode 5% of the movie (100 out of every 2000 frames).
3) Look at the resulting bitrate and reduce resolution accordingly, if the bitrate a lot different from your desired bitrate.
As described in the second link I posted there are some problems with that, so it's not as easy as just reducing the number of pixels by the same percentage as the bitrate you want to save, but at least it's a start.