OK Tony, you can go to 10 gpm on the 3/4 if it is class 200. Over that you'll exceed a 5ft/sec velocity. Velocity, thats a big word. Tony do you think some people can say velocity? If your sched 40 3/4" your down to 8 gpm to stay under 5 ft/sec. I think valve closing is related to manufacture. Some are just better than others. Some manufactuers recommend closing the "flow control" till head performance visually diminishes and then open up one turn.