The design and later use of modern spectropolarimeters and magnetographs require a number of tolerance specifications that allow the developers to build the instrument and then the scientists to interpret the data accuracy. Such specifications depend both on device-specific features and on the physical assumptions underlying the particular measurement technique. Here we discuss general properties of every magnetograph, such as the detectability thresholds for the vector magnetic field and the line-of-sight velocity, as well as specific properties of a given type of instrument, namely, that based on a pair of nematic liquid crystal variable retarders and a Fabry-Pérot etalon (or several) for carrying out the light polarization modulation and spectral analysis, respectively. We derive formulae that give the detection thresholds in terms of the signal-to-noise ratio of the observations and the polarimetric efficiencies of the instrument. Relationships are also established between inaccuracies in the solar physical quantities and instabilities in the instrument parameters. Such relationships allow, for example, one to translate scientific requirements for the velocity or the magnetic field into requirements for temperature or voltage stability. We also demonstrate that this type of magnetograph can theoretically reach the optimum polarimetric efficiencies of an ideal polarimeter, regardless of the optics in between the modulator and the analyzer. Such optics induce changes in the instrument parameters that are calculated.