Kerr rotation, theta K and ellipticity in K of magnetic thin films are measured using an optical set-up including a photoelastic modulator in series with a Babinet-Soleil compensator (phase shift gamma ) and an analyser (adjustable orientation beta ). According to Jones matrix analysis two equivalent calibration procedures of the theta K and in K values are proposed. In one method these may be determined by controlled tuning of beta and gamma respectively in the conventional compensation mode. On the other hand, in a novel two-angle method, they directly emerge from intensity ratios recorded at two distinct settings of beta and gamma respectively. Exemplary data on alloyed films of MnBi(Al,Si) are presented.