  We compare the extraction of the ground-state decay constant from the
two-point correlator in QCD and in potential models and show that the results
obtained at each step of the extraction procedure follow a very similar
pattern. We prove that allowing for a Borel-parameter-dependent effective
continuum threshold yields two essential improvements compared to employing a
Borel-parameter-independent quantity: (i) It reduces considerably the
(unphysical) dependence of the extracted bound-state mass and the decay
constant on the Borel parameter. (ii) In a potential model, where the actual
value of the decay constant is known from the Schroedinger equation, a
Borel-parameter-dependent threshold leads to an improvement of the accuracy of
the extraction procedure. Our findings suggest that in QCD a Borel-parameter
dependent threshold leads to a more reliable and accurate determination of
bound-state characteristics by the method of sum rules.
