Control valves are normally ...

Control valves are normally rated by a function referred to as Cv.

Cv is calculated (in imperial units) as:

Cv= Flow * sort( SG / DP)

where : flow (gallons per minute), SG = Specific Gravity, DP = Differential Pressure (pounds per square inch). 

I am certain you can find equivalent formulas using metric units.

So if you need 300 psi on the concentrate side of the membrane and have a concentrate specific gravity of 1.02, you can easily determine the Cv required for the valve to handle this flow.

I typically calculate both a wide open (very low DP) value and a normal operation condition (relatively high DP) and then start looking  for a valve that will provide a full open Cv to handle the low DP condition and an 70-80% closed position that will handle the high DP situation. In some cases you will not be able to get a valve that will open enough to allow the low DP condition and you will need to consider adding a bypass valve for low DP conditions such as membrane rinses and cleaning cycles.