Science deals with complex problems by simplifying them.
Earth is huge, has oceans and landmasses and mountains, its atmosphere is not the same everywhere, water evaporates and condenses into rain or snow all the time, winds and water currents influence temperature distribution, and the most important : et cetera. Complex as hell (or more so).
I say Earth is a perfect sphere of a given radius and mass, with an homogenous layer of static gases around it. NOW I can determine the theoretical greenhouse factor. No climate, no vegetation favorised by CO2, no oceans and landmasses. I barely acknowledge the planetary albedo, which encompasses snow, ice, water, land and cloud coverage. For me, this is what Aurora Terraforming is about.
You want to complexify it? Add a tiny bit to the simple model : tectonic activity indicates nuclear reactions in the planet's core, heating it enough to keep it liquid, and so increasing slightly surface temperature. Another tiny bit : by using reflectors, we can reduce or increase exposure to solar radiation, thus in a way modify planetary albedo.
This is lightyears from climate and vegetation, there's not even geography involved!