Actually, though, most of the OLR originates from below the tropopause (can get up around 18 km in the tropics, generally lower)-- with a majority of solar radiation absorbed at the
surface, a crude approximation can be made that the
area emitting to space is less than 2 * (20/6371) * 100 % ~
= 0.628 %
more than the
area heated by the sun, so the OLR per unit
area should be well within about 0.6 % of the value calculated without the Earth's curvature (I'm guessing it would actually be closer to if not less than 0.3 % different).