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The 200 psig needs to be 'set' by your downstream piping and the hydraulics in that system.
Let's say you had a control valve downstream of your RO controlling the pressure. You could set that control valve at any pressure less than the pressure that gives you the 'choked' flow across your RO with no change in flow through the RO. The control valve has set the dP for your RO, not the RO itself.
The RO is sized for a given flow and pressure drop (or part of the total pressure drop when you have choked flow) but it doesn't really 'create' that dP in the system.
I know I'm not explaining this well. Basically, wherever the flow that goes through the RO goes to, that downstream system and whatever controls is on it sets your downstream pressure, not really the RO itself (a sketch might help you here). Let's say you had a control valve right downstream of the RO. YOu could change the downstream pressure on the RO to any pressure less than the dP corresponding to your choked flow without affecting the flow through the orifice. If you put in a bigger orifice, you still wouldn't change the pressure downstream of the RO as long as the control valve had sufficient capacity to handle the additional flow through the RO.
You need to look at the system hydraulics when thinking about the DP the RO will take. It's similar to a control valve in that the control valve doesn't 'take' a pressure drop, it has a pressure drop for it established by the system it is installed in.