Calculate transmission / reflectivity instead of absolute quantities. For irradiances / actinic fluxes the transmission T is defined as

T={{E}\over{E_{0}\cos{\theta}}} (1)

where E is the irradiance / actinic flux, E_{0} is the extraterrestrial flux, and \theta is the solar zenith angle. The reflectivity R is defined as

R={{\pi\cdot L}\over{E_{0}\cos{\theta}}} (2)

where L is the radiance, E_{0} is the extraterrestrial flux, and \theta is the solar zenith angle. Obviously, reflectivities do not depend on Sun-Earth distance. Please note the difference to transmittance.