\displaystyle c_\phi(p) = c_{\phi i} \cdot \frac{ \ln \left( \frac{ p_n }{p_{\rm max}} \right)  }{ \ln \left( \frac{p_{ni}}{p_{\rm max}} \right)  }

p_n = p_{\rm min} + 1.75 \cdot \phi^{0.51} \cdot (p_{\rm max} - p) 

p_{ni} = p_{\rm min} + 1.75 \cdot \phi^{0.51} \cdot (p_{\rm max} - p_i)