The illumination uniformity of a spherical target is of fundamental importance in direct-drive inertial confinement fusion. The laser intensity asymmetry is studied here using numerical calculations, taking into account target positioning errors and laser system shot-to-shot fluctuations. The optimisation of a laser beam configuration is outlined on the basis of a statistical analysis of the illumination uniformity using three independent selection criteria. The particular case of a 48-beam laser facility is discussed in detail. It is found that the fluctuation of the illumination asymmetry is a key parameter for assessing the stability of operation in high-repetition-rate mode. Moreover the target positioning error arising from the injection system could be the dominant source of illumination asymmetry in a fusion reactor. Copyright (C) EPLA, 2011