  The control of material loss mechanisms is playing an increasingly important
role for improving coherence times of superconducting quantum devices. Such
material losses can be characterized through the measurement of planar
superconducting resonators, which reflect losses through the resonance's
quality factor $Q_l$. The resonance quality factor consists of both internal
(material) losses as well as coupling losses when resonance photons escape back
into the measurement circuit. The combined losses are then described as
$Q_l^{-1} = \mathrm{Re}\{Q_c^{-1}\} + Q_i^{-1}$, where $Q_c$ and $Q_i$ reflect
the coupling and internal quality factors of the resonator, respectively. To
separate the relative contributions of $Q_i$ and $Q_c$ to $Q_l$,
diameter-correcting circle fits use algebraic or geometric means to fit the
resonance signal on the complex plane. However, such circle fits can produce
varied results, so to address this issue, we use a combination of simulation
and experiment to determine the reliability of a fitting algorithm across a
wide range of quality factor values from $Q_i\ll Q_c$ to $Q_c\ll Q_i$. In
addition, we develop a novel measurement protocol that can not only reduce
fitting errors by factors $\gtrsim 2$ but also mitigates the influence of the
measurement background on the fit results. This technique can be generalized
for other resonance systems beyond superconducting resonators.
