One-loop SU(5) prediction with common flavor coefficients (${\sin }^2{\theta }_W=\frac{1}{6}+\frac{5\alpha }{9{\alpha }_s}$)

Normalize the Abelian coupling by $g_1^2=(5/3)g^{\prime 2}$ and match $g_1=g_2=g_3$ in magnitude at scale $M_X$. With common-flavor coefficients $b_3=(33-2n_f)/3$, $b_2=(22-2n_f)/3$ and $b_1=-2n_f/3$, the one-loop running of the strong coupling extends to $1/{\alpha }_i(\mu )=1/{\alpha }_5-b_i\log (M_X^2/{\mu }^2)/(4\pi )$. The differences obey ${\alpha }_1^{-1}-{\alpha }_2^{-1}=2({\alpha }_2^{-1}-{\alpha }_s^{-1})$. Using ${\alpha }_2^{-1}={\sin }^2{\theta }_W/\alpha$ and ${\alpha }_1^{-1}=3{\cos }^2{\theta }_W/(5\alpha )$ gives the displayed prediction and $\log (M_X/\mu )=\frac{\pi }{11}({\alpha }^{-1}-8/(3{\alpha }_s))$. These simplified coefficients omit the full matter and scalar counting of a realistic unified model; threshold changes require piecewise running with the actual coefficients.