A method of estimating the margin of error for Thomson scattering systems based on polychromators has been devised during the operation of the Translation, Confinement, and Sustainment Upgrade (TCSU) experiment. This method first uses the propagation of uncertainty to determine the standard deviation (SD) of the ratio between two output signals. Later the SD or error is projected onto a characteristic curve that relates different ratios of the signal output to the electron temperature. This method brings an asymmetry to the error bounds, which goes accordingly to the ratio of the spectral response function for distinguishing higher temperatures. Also, the method follows with the nature of photon-statistics. As the plasma density is increased, as one might expect, the corresponding amplitude of the error bar becomes smaller.