In Mott polarimeters, the measurement of the spin polarization of an electron beam along two perpendicular quantization axes is commonly performed by collecting scattered electrons with four independent detectors placed azimuthally at 90° from each other. However, the four intensities are not independent quantities. This overlooked redundancy will be pointed out, and it will be shown how to use it in order to better design, calibrate, and operate Mott polarimeters.