\(\varphi_{\text{hnf}}=\frac{X_{1}\left(\frac{M}{\rho}\right)_{1}+X_{2}\left(\frac{M}{\rho}\right)_{2}}{X_{1}\left(\frac{M}{\rho}\right)_{1}+X_{2}\left(\frac{M}{\rho}\right)_{2}+\left(\frac{M}{\rho}\right)_{\text{bf}}}\) (4)