  We investigate the theoretical predictions for thrust distribution in the
electron positron annihilation to three-jets process at NNLO for different
values of the number of flavors, $N_f$. To determine the distribution along the
entire renormalization group flow from the highest energies to zero energy we
consider the number of flavors near the upper boundary of the conformal window.
In this regime of number of flavors the theory develops a perturbative infrared
interacting fixed point. We then consider also the QED thrust obtained as the
limit $N_c \rightarrow 0$ of the number of colors. In this case the low energy
limit is governed by an infrared free theory. Using these quantum field
theories limits as theoretical laboratories we arrive at an interesting
comparison between the Conventional Scale Setting - (CSS) and the Principle of
Maximum Conformality (PMC$_\infty$) methods. We show that within the
perturbative regime of the conformal window the two methods yield similar
results while reducing the number of flavors towards the active number of
flavors in the Standard Model the PMC$_\infty$ method is the natural extension
of the curves within the conformal window and best fits the data.
