The definition of impedance is not unique for the high-frequency millimeter wavelength range. This work strives to demystify the difference between the three impedance calculation approaches in practice (power-current method, power-voltage method, and voltage-current method) based on a case study into 60 GHz liquid crystal enabled tunable delay lines structured in a coaxial topology. For a same coaxial geometry with the same dielectric thickness of liquid crystal (at saturated bias state), the quantified results of characteristic impedance by the power-current and voltage-current methods agree closely, whereas the power-voltage method exhibits a lower prediction, albeit the deviation is within 0.4 ohms. Getting a full knowledge of the precision of impedance calculations under lossy line conditions paves the way for liquid crystal-based reconfigurable millimeter-wave applications, in particular, impedance adapters and tuners.