FWIW, I now have a lot of experience with the Parker metering valve. using 20 psi into both the fluid and air ports, I find that the setting for the metering valve which produces the best flow (very slightly wet) is still tricky - very close to fully closed, BUT, and it's a big BUT, it can be found and once set seems to hold through a run, which for my jobs can be an hour or two. It's pretty hard to see the actual output of the tube at this setting, but what I've been doing is adjusting until I see some fluid coming out the tube, then reducing the flow until i still see fluid accumulating on workpiece but cannot see stream itself. I've mostly been cutting aluminum and get good finishes and no chip-sticking. chips do clump up though and I have to blow them away from time to time.
It's amazing how little fluid gets used in two hours yet there's enough to see on the workpiece.