Atmosphere must be different in your neck of the woods then. 28" H20 seems to be just right IME. If the head isn't a real big air mover I'll flow it @ 55" H20 even and the conversion always lines up--IF there isn't a stall. It's one more way IMO to "tell" if a port is going to perform well. Heck, I've seen the opposite you have at least a couple of times where the port got slightly turbulent @ 28" but held on @ 50". J.Rob