Thank you for the comments. Much appreciated.
Just a couple of points:
1. About control.
The screw thread for the valve control is an M8 coarse thread (1.25mm pitch) so for each turn the inside cone moves 1.25 mm. If one looks at top picture you will notice a "clock face" with 5 marks. Each mark represents a movement of 0.25mm (4 marks = 1.0mm). AS one can see this valve gives very fine control. The brass knob has a reference mark (not visible in the picture) engraved in it. Depending on the inside cone the total movement may be 20mm (in 0.25mm steps !!)
The valve area can go from 2xArea of the 1" exit tube to the area of the exit tube.
2. About the Outside cone.
Two matching cones ensure that the valve area tapers down in a "perfectly" linear way. My research, by looking at designs on various web forums, showed that this matching of apertures is "quite a problem".
This discussion is very, very good :-
https://aussiedistiller.com.au/viewtopic.php?f=8&t=7180