The aim of this work is to help for choosing the optimal configuration of the VLTI for the observation of dust shell inner boundary of the S star π1 Gru. We show that this depends on the choice of the used geometrical model. The visibility curves produced by different models are compared to that predicted by the DUSTY spherical transfer radiative numerical code fitting real wide-band spectrophotometric data. The JMMC-ASPRO software is used to calculate the visibility points from the DUSTY code. The optimal configuration of the VLTI with 2 and 3 auxiliary telescopes, depending on the used geometrical model, minimizes the error on the inner shell radius.