Lets say that you have 0% throttling (this is 100% open valve), then 20, 40, 60, 80% throttling...following your description...100% throttling would be shut-off valve.
You may indeed draw 5 different curves (for 0/20/40/60/80% throtling), the 100% throttling would be the vertical axis (and your 6th curve).
Of course there are infinite number of system curves depending on the throttling.
The Resistance coefficient K for the control valve @ different positions is usually included in the manufacturer's data (flow vs. press drop, Cv, etc).
A very crude approximation to K would be using Crane TP410 fig in page A-31 if you know the Cv of the valve at different throttling positions.
Also, in case you do not have valve data you could just plot the 0% throttle considering the press drop across the fully open valve (e.g. using Crane TP410 K-factor Table (starting on page A-26). This curve will intersect the pump curve at Qz.
Calculate 0.8Qz, 0.6Qz, 0.4Qz, 0.2Qz
Determine delta P from the pump curve for those flows.
Each curve could be now calculated from (e.g. for 0.8Qz)
delta P = (delta P@0.8Qz / (0.8Qz)^2)* Q^2
The difference between the delta P where each curve intersects the pump curve, and the curve for 0% throttling is the pressure drop required at the control valve at each of the p*Qz flows, where p = 0.8, 0.6, 0.4, 0.2
With the flow and pressure drop you may be able to determine a suitable control valve, but consider that the best range for operation of a control valve is between 60 to 80% of the full range... therefore when selecting the control valve you may want to consider only the flow and pressure drop for 20 and 40% throttling and then evaluate the other operating conditions... e.g. for a system where precise control is required at low flows you may even consider a shut-off valve for the "big valve", the one we just defined and a by-pass around that valve with a "small control valve" that allows control in the lower flow ranges.
Two notes:
1. Beware of the high flows... high flows may cause the pump to cavitate (if the impeller is too big for the design flowrate of the system) - i.e. your flow is to the right of the flow where the pump cavitates.
2. Beware of low flows, do not go to the left of the minimum flow for the pump as recommended by the pump manufacturer
Hope this is what you were looking for.
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