The average flow model proposed by Patir and Cheng offers a great convenience for the mixed-lubrication analysis of rough surfaces. The pressure flow factor introduced by Patir and Cheng helps to analyze the influence of roughness on the average pressure and average flow of a lubricant between surfaces. This paper reports how to reasonably choose initial pressure in computing the pressure flow factor. The numerical results show that the pressure distribution and further pressure flow factor value are sensitive to an initial pressure in solving the pressure flow factor, and the initial pressure of a constant value is not suitable for the pressure flow factor calculation for a measured surface with many sample points. Meanwhile, the pressure flow code is demonstrated by the comparison of the pressure obtained numerically with the analytical solution of pressure.