You need to look at the system to calculate the pressure drop across a control valve.
Take an easy example, letting down steam from 200 psig to 50 psig. If the piping is short, then the pressure drop is always going to be 150 psi regardless of the flow rate (for this purpose, I'm ignoring variations in the upstream or downstream steam pressure).
Another example could be pumping product from a unit to storage. The destination pressure will vary depending on the level in the storage tank. The line losses through piping and equipment will set the outlet pressure on your control valve. For the maximum outlet pressure I would typically use the max flow and resulting pressure drop through the equipment/piping and a full tank. For the minimum outlet pressure I would then use the turndown flow and resulting pressure drop at say a close to empty tank. If this is a new design, then you need to decide how much pressure you want to take across the pressure drop at the maximum flow case. I find this approach gives me a good estimate of the maximum sizing case, normal sizing case and minimum sizing case. The maximum and minimum sizing cases are the most important, if you define those correctly for your instrument engineer the selected valve will handle any intermediate case which will include the 'normal' case.
The dP cant be "calculated" is actually specified by your process and then you calculate the required capacity of the valve to meet your flow requirement. And its not just "splitting hairs". Its a way of thinking!
Explanation (from upstream oil&gas production): You have two separators and with to obtain a certain oil specification, This requires that the pressure of your last separatoris 0.25 barg. Your process simulation and cost optimizations finds that the upstream separator must have a pressure of 10 barg. So here the dP of the CV is _given_ by the process (9.75 bar) and you cannot "calculate" this. In order to find the max Cv of the valve you may say that at the nominal flow you with the valve to be 70% open in order to obtain good control. Then you assume someting about the trim, find the required max CV and then find your valve.
