We report on numerical studies of phase front propagation for a large-baseline space interferometer. The main objective is to determine the sensitivity of the average phase of the metrology beam with respect to fluctuations in the pointing of the beam. For this purpose, the metrology beam is propagated numerically along the interferometric arm of the instrument. The effects of the obscurations from the secondary mirror and its supporting struts are studied in detail. Further, the effects of wavefront distortions that occur due to imperfections of the optical elements are estimated through a series of Monte Carlo simulations. The results of this study can be used to determine design requirements for the instrument.