I am not quite sure I am catching your question, please correct and specify if I am wrong!
If you are talking about a proportioonal operated solenoid valve, or a similar valve where flow is proportional to a regulating signal, for instance a signal in range 0-20mA, you have to divide between the operating signal (0-20mA) and the necessary operating force to balance the opening.
The necessary force is proportional to the opening degree (0-20mA) but also proportional to the pressure for a direct operated proportional valve. Higher liquid pressure will give higher necessary force at a given opening.
(If you have a balanced operating valve the force is more constant, but anyway you need to change and balance the opening degree)
Most often the input fluid pressure to the valve is kept (relatively) constant, and you use the operating signal to give (balance)an opening.
The valve will then give a (relatively) constant flow at a given opening. If you change the input signal, you change the opening and hence the flow proportionally.
The valve is balanced at this opening internally by an internal position signal, given from the piston position, compared to the incoming operating signal.
If you have this valve construction and increases the incoming pressure, the flow will increase, if the ingoing operating signal and everything else is kept constant.
To lower the flow to the wished flow value you need extra force to balance the pressure, keeping a somewhat smaller opening.
For such cases you need your input operating signal to the valve to change (increased force, lesser opening) until correct flow is obtained.
You can obtain this by making the input operating signal as a processed result from a flow measuring or pressure measuring device downstream the valve.
