In this paper, we devise a position-invariant method for unique and accurate complex permittivity (<inline-formula> <tex-math notation="LaTeX">\varepsilon _{r} </tex-math></inline-formula>) determination of low-loss samples from transmission and shorted-reflection scattering (S-) parameter measurements while mitigating the effect around Fabry-Perot frequencies. For this goal, we derived a metric function in terms of propagation factor <inline-formula> <tex-math notation="LaTeX">T </tex-math></inline-formula> only and utilized a branch-index-independent expression for unique <inline-formula> <tex-math notation="LaTeX">\varepsilon _{r} </tex-math></inline-formula> by eliminating multiple solutions problem. We measured S-parameters of two low-loss samples with substantial thickness, which both introduced a Fabry-Perot effect in the frequency range, and the measurements were conducted to validate our method and compare its accuracy with the accuracy of similar methods in the literature. We also performed an uncertainty analysis to evaluate and improve the accuracy of our method.