It’s an interesting exercise to calculate the two components of irradiance as a function of Venus’s angular distance from the sun: the 1/r^2 component derived f

It’s an interesting exercise to calculate the two components of irradiance as a function of Venus’s angular distance from the sun: the 1/r^2 component derived from the Earth–Venus distance, and the geometric factor that depends on the angle of illumination. Combining these and finding the maximum, you should get the result mentioned.