That's what I was getting at: how the internal passages are drilled. the size of the orfice sets the absolute flow rate of the oil. How quickly the valve goes from closed (no oil flow) to wide open (max flow allowed by the orfice) is controlled by the thread pitch (how far the valve opens for a given rotation of the screw) and the amount of area for the oil to flow through, How quickly the area is varied is determined by the taper of the valve and the orfice, like the idle mixture screw in a carburetor. At least that explains it as well as I can if my understanding is correct.
Meanwhile, back at the original situation, I think I have a much simpler solution. If I clamp vise grips crossways on the shaft I can change the valve opening more gradually. That's what I think BadSport suggested several posts back. I've been making a mountain out of a molehill!! DUH!!!
