I would do it by measuring the amount of radiation leaveing the surface compared to the amount detected from orbit. Then calculate the power per molecule by calculation, using the PPM of carbon dioxide, and pressure in different parts of the atmosphere.
Hmm, that probably makes more sense. Less susceptible to error. The absolute magnitude of Venus should be something you can look up. And then from that you can calculate a flux coming off the planet (maybe there's a formula somewhere?).
There wouldn't happen to be a calculation of the mass of the atmosphere of Venus and Earth anywhere, is there?
...
All that said, it's a very rough approximation. The hard part I think is determining what the temperature would have been without any greenhouse effect. Because the greenhouse effect is basically an albedo term for lower frequencies. Meaning that everything in an atmosphere has some effect on the "greenhouse effect" just from its absorption/transparency/reflection of different colors of light.
CO2 is transparent to visible light and more opaque to infrared(? Or is it even lower frequencies of light?), so it lets the majority of the solar flux (which is centered at visible light) in. And then that flux heats the ground or water, and that ground and water release lower frequency light back out from its black body radiation. And CO2 absorbs some of that reflected light, heats up, and releases even lower frequencies back to the ground or into space.
So the final temperature is a steady state involving the black body radiation of the planet coupled with the fact that a planet isn't a perfect black body and has all sorts of albedo terms from clouds and the ground, and the ground itself isn't homogeneous and has all sorts of its own albedo terms. And these terms are temperature dependent. Solid rock has a different albedo from magma, for instance.
So a real term is understandably difficult to arrive at.