System consists of a cylinder (piston), pressurizing fluid through a mechanical flow control valve in the restricted direction, (metered flow in 1 direction, free flow in oppositte direction), pouring into a resevoir exposed to atm press.
The system was tapped with two pressure gages immediately before and after the metered control valve. Operating the piston gives me a gage reading of 1000psi just before the valve and 0psi just after the valve. I'm assuming the 0psi  is due to zero back pressure (gage) at the resevoir therefore the pressure head is in the form of (V^2)/2g.
Using the Bernoulli equation, you can see you have an overdetermined system, i.e. not enough unknowns.  If you know the inlet pressure and the flow, you must calculate (or physically adjust) the outlet pressure.  If you know the outlet pressure and flow, you must calculate the inlet pressure.  If you know both inlet and outlet pressures, you must calculate flow.  Here you are simply physically adjusting the outlet pressure (backpressure) to correspond with your gauge observations and the mathematical results predicted by the Bernoulli equation given a known inlet pressure, a known flow and an unknown outlet pressure.  In this case the known flow could be determined from the maximum possible flow across the valve, Q = Cv x dP^2/SG^0.5   You have thus determined that the outlet pressure must be 800 psi (or less) to match the inlet pressure and this valve's maximum flowrate for that system.  You have also reached the valve's maximum flow capacity at your particular %open when you have a 200 psi differential pressure across the valve and you have found that any more pressure drop will not accelerate the flow nor increase the actual pressure drop across the valve any more than 200 psi you already have.

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