I actually don't have a clue.Speaking from my own experience, those figures (+300 to +400mV) are not easy to achieve. I suspect there are many tanks out there that fall short of this range. All it takes is a small amount of organic waste to drag these figures down towards the +200mV mark.
I tested mine for fun, and got 300+. How accurate that is or how the numbers are just before waterchange, is unknown.
Once I get my CO2 dialed in, I can switch probes and do a profile. Just for fun