We report a significant improvement in the measurement accuracy of electro-optic (EO) coefficients for low-loss EO polymers on substrates of sol-gel silica and indium tin oxide (ITO). Initially, we apply the standard Teng and Man reflection ellipsometric method, which results in substantial variability in the measured EO coefficients across a wavelength spectrum with changes as small as <1 nm. This variance leads to unreliable EO coefficient values ranging from a few to 70 pm/V at the 1.31 and 1.55 µm wavelengths. By adopting a transmission method for our experiments, we effectively mitigate the dependence of the measured EO coefficient on the wavelength variance of 0.2 nm. As a result, this new approach enables a more accurate and reliable measurement of the EO coefficients. This breakthrough presents a significant step forward in the field of EO research, paving the way for further exploration into the behavior and properties of EO polymers. Additionally, our findings highlight the importance of selecting an appropriate measurement method in accordance with the unique properties of the material under investigation.