The IPCC statement of a 1.2 K rise is http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/044.htm

A quick back-of-envelope calculation is that dE/DT = 1/[4sigmaT^3] whee dE is the radiative forcing for a doubling, roughly 5.35 ln 2 (Myhre et al 1998). Let's say 4 W/m2 per doubling and 4sigmaT^3 is the slope of Stefan-Boltzmann.

or, 4/4sigmaT^3 where T is the radiating temperature of the planet (not the surface) which is 255 K. That gives roughly a 1 K rise (not F).