I'm trying to create my own porous media to control airflow so I need to be able to determine the pressure drop vs velocity graph. Does anyone have any way to know how to calculate these values?

What I also find interesting is that in the Porous Media > Pre-Definded > UAF > Metal Mesh Filters, there is three (3) different options, 1/2'', 1/4'' and 5/8''. To start with the 1/4'' has a a low pressure difference, this is then roughly doubled in the 1/2'' option but strangely enough the 5/8'' option (which is the thickest) has the lowest pressure difference of all. 

Am I missing something obvious or is that extremely strange, can someone explain? Thanks.