A dual-rotating-retarder polarimeter was used to determine the six measurable observables of the first hyperpolarizability tensor. Calibration of such an instrument requires a reference sample dedicated to wavelength conversion. We calibrated our experimental setup by using a quartz-plate sample in a two step procedure: at first the first retarder then the second one. The retardance and ellipticity angle of both retarders were estimated by minimizing a chi(2) function. We estimated the standard deviation of each parameter from noise spreading and performed this calibration procedure for two experimental case studies, i.e., two angular positions of the quartz sample.