  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 and also out of the conformal
window the PMC$_\infty$ leads to a higher precision, and that reducing the
number of flavors, from the upper boundary to the lower boundary, through the
phase transition the curves given by the PMC$_\infty$ method preserve with
continuity the position of the peak, showing perfect agreement with the
experimental data already at NNLO.
