There are 30 closely located vertical irradiation channels in the beryllium reflector of the SM high flux reactor. Based on "exact" Monte-Carlo numerical neutron-physical calculations, a simple analytical approach was developed. This method allows one to estimate the neutron flux density on any channel, taking into account both its own absorbing loading and water replacement and perturbation on the nearest channels. The computer code FLUXSM was developed based on the proposed approach. This code has been used for calculation of different optional macroloadings of the SM reflector channels, and our experience with it demonstrated the high reliability of the results.