The crystallographic texture of an asymmetrically rolled sheet was investigated by electron back scattered diffraction (EBSD) and exhibited prismatic monoclinic symmetry. The parameters of the associated yield equation, derived from the quadratic orthotropic Hill criterion, were determined from the volume fractions of the texture components using the "Continuum Mechanics of Textured Polycrystals" (CMTP) approach. Two quantities are defined to assess the departure from orthotropy, and a method is proposed to find the "best" orthotropic estimation of the sheet behaviour. Results are first discussed on the basis of strain rate ratio predictions. Mechanical behaviour analysis of a sheet submitted to combined shear and compression (CSC) loading is also used to illustrate the method.