How do I find the critical flux of a CTA membrane that I am to use for a reverse osmosis operation?

In most of the websites, it is said that it is a value found out by trial and error or by experience in the field. There seems to be no definite quantifiable technique or equation. I cannot correctly guesstimate the flux without knowing the critical flux and advance the calculations further. As someone who is simply designing the operation specs with no access to the experimental setup, what can I do to find it out?