  A complete calculation of the ${\cal O}(\alpha_s^4)$ perturbative QCD
corrections to the hadronic decay width of the $Z$-boson has recently been
performed by Baikov et al.[1]. In their analysis, Baikov et al. relied on the
conventional practice of simply guessing the renormalization scale and taking
an arbitrary range to estimate the pQCD uncertainties. This procedure
inevitably introduces an arbitrary, scheme-dependent theoretical systematic
error in the predictions. In this paper, we show that the renormalization scale
uncertainties for hadronic $Z$ decays can be greatly reduced by applying the
principle of maximum conformality (PMC), a rigorous extension of the BLM
method. The PMC prediction is independent of the choice of renormalization
scheme; i.e., it respects renormalization group invariance, and thus it
provides an optimal and theoretically rigorous method for setting the
renormalization scale. We show that the convergence of the pQCD prediction for
the $Z$ hadronic width is greatly improved using the PMC since the divergent
renormalon series does not appear. The magnitude of the high-order corrections
quickly approach a steady point. The PMC predictions also have the property
that any residual dependence on the choice of initial scale is highly
suppressed, even for low-order predictions. Thus, one obtains optimal
fixed-order predictions for the $Z$-boson hadronic decay rates thus enabling
high precision tests of the Standard Model.
