The rationale associated with the first if-condition hinging around not allowing the diminishment of stiffness is the thing I could not grasp for sure. In this light, have you ever seen any references putting forth the aforementioned criteria?
My gut feeling is that the amount of Pref otta be firstly perused. It is common to assign 100 kPa to the aforesaid parameter named "reference pressure". Are we demanded to make use of 100 kPa in every projects? In addition, what does (P/Pref