But I SITLL want to know how the system "sees" the flow - how does it KNOW it has too much or too little?

And does the P0400 mean there is an indication of too little flow, or it is more of a "something may be wrong because...."? I just want to know, because I wonder if the problem is that a sensor is bad and LYING to the PTCM. Because in that case, replacement of the EGR valve assembly would be a waste of money and time.