Sugestions for tool i'm making -> How to estimate resolution Vs Bitrate

[OvejaNegra]

As the title says. I'm making a batch tool for x264. I know there are not absolute values for any source, but sometimes you can have (maybe) an estimated resolution VS bitrate value, so you can estimate how much bitrate you need for a certain resolution.

Yes, i know there are not absolute value for this.
¿But how autoGK does that?
I was reading this:
http://www.mplayerhq.hu/DOCS/HTML/en...itrate-compute

but it was not too clear for me.

Is there any "general use" method to estimate the bitrate for a given resolution? (or to chose a resolution based on available bitrate)?

I know some people make a compressibility test, to estimate the optimal bitrate for some sources ( i think it's done with constant quantizer) but i don't know how.

Any suggesions
thanks