\(\frac{k_{\text{hnf}}}{k_{\text{bf}}}=\left(\left(k_{1}X_{1}+k_{2}X_{2}\right)+2k_{\text{bf}}+2\left(\varphi_{1}k_{1}X_{1}+\varphi_{2}k_{2}X_{2}\right)-2\varphi_{\text{hnf}}k_{\text{bf}}\right)\) (3)