What is the ratio of infra-red light absorbed by green house gases?
There is no simple relationship since it all depends on the frequency (IR spectral lines for most species of molecules are a mess). The most direct and precise way of calculation is through line-by-line calculation from a large spectrum database. The atmospheric column will be very different based on angle off normal, weather, and even time of day (water content has diurnal variability, mostly).

Surface temperature and surface properties also influence the distribution of emitted IR.

See also: Atmospheric radiative transfer codes