Bankability of Solar Tower Plant (STP) projects requires the best estimates of the solar resource. For that reason, several studies can be found on the reduction of estimates uncertainty. Currently the solar resource assessment of STPs is usually separated into two parts: 1) The estimation of the Direct Normal Irradiance (DNI) from ground-based measurements or from Radiative Transfer (RT) codes. 2) The simulation of solar radiation, with DNI as an input parameter, between heliostats and the receiver from Optical Simulation codes (i.e. DELSOL3, STRAL, SolTRACE, etc). Instead of stopping the solar radiation simulation at ground level for the estimation of DNI, the RT code SMART-G [Ramon et al., 2019], considered now as a reference for RT simulations, can continue the simulation by considering the heliostat reflections, to estimate the solar resource directly at the receiver. Such a direct simulation became possible thanks to the exponential improve of the High-Performance Computing, where here it is the Graphics Power Unit technology for the parallelized code SMART-G. By simulating directly the solar radiation from the sun to the receiver [Moulana et al., 2018] have shown that it is possible to consider now the collected gain contribution from the diffuse solar radiation, enabling the improve of the solar resource accuracy. The gain contribution was estimated up to 5% for a simplified system of maximum 8 heliostats [Moulana et al., 2018]. Such a contribution is not negligible. Therefore, we go further by presenting here more realistic gain contribution estimates, from a complete STP of hundreds of heliostats. This consideration allow to take into account the shadow and blocking losses [Li et al., 2016]. A sensitivity study is presented for different Aerosol Optical Thickness (AOT) values but also for different sun zenith angles.