For the ITER experimental fusion reactor up to 3 heating neutral beams (HNBs) are foreseen. Each HNB will generate 1280 individual beamlets of H-/D- ions which will be accelerated up to 870 keV 1 MeV−1, neutralized and focused into the tokamak plasma, to provide up to 16.5 MW of heating power. To this purpose, divergence and aiming of each beamlet are required to be ⩽7 mrad and within ±2 mrad, respectively. An accurate compensation is thus essential, both for the magnetic deflection induced by the magnets embedded in the extraction grid (EG) (necessary to immediately deflect the co-accelerated electrons) and for the Coulomb repulsion among the beamlets. The Asymmetric Deflection Compensation Magnets, the solution developed for the full-scale ITER HNB prototype called MITICA, was recently tested, for the first time on a MeV class beam, at the MegaVolt Test Facility (MTF) at QST (Naka, Japan). In these experiments, a MITICA-like EG was built and installed on MTF. In this work, the beam pattern measured on a calorimeter made of unidirectional carbon fibre composite is compared with the predictions from simulations by the numerical code IBSimu, in order to develop methodologies and identify limitations for its application for MITICA exploitation and improvements. The model accuracy varied with experimental conditions; in all the cases, however, the proposed methodology proved very valuable in providing trends and dependencies.