If you want to reduce the pressure downstream to a preset value, the valve is a pressure regulator and the pressure tap (internal or external) is locate downstream. Lots of manufacturers provide this type of PRV ; see Emersonprocess (formerly Fisher) model 95 for example.
If you want to control a constant flow rate donwstream independently of upstream pressure, the valve is a differential pressure regulator ; 2 pressure taps is needed (upstream and downstream). See Jordan valves (model 63).
MDP