We present results for the thrust distribution in the
electron positron annihilation to the three jet process at NNLO in
the perturbative conformal window of QCD, as a function of the
number of flavors $N_f$. Given the existence of an infrared
interacting fixed point in this region, we can compare the
Conventional Scale Setting (CSS) and the Principle of Maximum
Conformality (PMC$_\infty$) methods along the entire
renormalization group flow from the highest energies to zero
energy. We then consider also the QED thrust, obtained as the
limit $N_c \rightarrow 0$ of the number of colors and we show
analogous comparison. QED in the low energy regime develops an
infrared non-interacting fixed point. Using these quantum field
theory limits as theoretical laboratories, we arrive at
interesting results showing new features of the PMC$_\infty$.