We present a new measurement technique based on reduced-reflectometer calibration of a waveguide with a highly-reflecting termination for the complex permittivity characterization of viscous materials. In this novel method, the waveguide is simply dipped into the viscous material at 4 different depths, separated roughly by $\lambda/8$, and the reflection coefficients are recorded. Subsequently, a multi-reflect calibration procedure is applied to these measurements to determine the propagation constant of material-filled waveguide. The complex permittivity of the material is then extracted from the calculated propagation constant. The proposed approach is very simple and cost-effective, as it only requires a one-port network analyzer, and is applicable to the wide range of frequencies. We show the validity of this new technique by characterization of olive oil over 90-130 GHz band.