  Interpolating functional method is a powerful tool for studying the behavior
of a quantity in the intermediate region of the parameter space of interest by
using its perturbative expansions at both ends. Recently several interpolating
functional methods have been proposed, in addition to the well-known Pade
approximant, namely the "Fractional Power of Polynomial" (FPP) and the
"Fractional Power of Rational functions" (FPR) methods. Since combinations of
these methods also give interpolating functions, we may end up with multitudes
of the possible approaches. So a criterion for choosing an appropriate
interpolating function is very much needed. In this paper, we propose reference
quantities which can be used for choosing a good interpolating function. In
order to validate the prescription based on these quantities, we study the
degree of correlation between "the reference quantities" and the "actual degree
of deviation between the interpolating function and the true function" in
examples where the true functions are known.
