Hi,
I have also tested the sliding plate system. When calibrated, the flow numbers are within 0.3 cfms from a range of 25 to 250. Tested with about 20 objects and five positions.
I had two plates with a round slot cut to 44.5 mm sliding against each other in a groove.
The plate was adjusted with a screw with position markers. Unfortunatly this could not be implemented without leaks in my orifice in pipe design.
/kimi