Comparison of the primary X-ray spectra in the point source approximation used in KYNSED (black lines) with the 3D corona Monte Carlo computations of MONK (colour lines) for the spherical corona with different sizes, small (magenta), medium (blue), and large (yellow), at three heights. The three radii of the corona are R_c = 0.5, 1.5, and 2 r_g for the height h = 3 r_g; R_c = 1, 6, and 9 r_g for the height h = 10 r_g; and R_c = 5, 15, and 29 r_g for the height h = 30 r_g. The corona with the largest size extends as far as the BH horizon. The luminosities are divided by so that we normalise the results to the same (unit) radius. See Sect. 5.3 for more details.