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In this paper, we present a new analysis on the P-wave charmonium annihilation into two photons up to next-to-next-to-leading order (NNLO) QCD corrections by using the principle of maximum conformality (PMC). The conventional perturbative QCD prediction shows strong scale dependence and deviates largely from the BESIII measurements. After applying the PMC, we obtain a more precise scale-invariant pQCD prediction, which also agrees with the BESIII measurements within errors, i.e. R = Γ γ γ (χ c 2) / Γ γ γ (χ c 0) = 0.246 ± 0.013 , where the error is for Δ α s (M τ) = ± 0.016 . By further considering the color-octet contributions, even the central value can be in agreement with the data. This shows the importance of a correct scale-setting approach. We also give a prediction for the ratio involving χ b 0 , b 2 → γ γ , which could be tested in future Belle II experiment. [ABSTRACT FROM AUTHOR]