HMI continuum image of the leading sunspot of active region NOAA 11072 (2010.05.25 03:00:00 TAI), which was used as a reference for the model presented in this work. The black solid line outlines the umbral area that was considered for most twist calculation methods. The white solid line and the black dotted line correspond to the areas that were used to get spatial averages of α_z close to the center of the spot () and over the whole spot (), respectively.