The data sheet for a choke valve. I know the fluid (gas) and the pressures of inlet and outlet. I have to give CV values required and the mass flow rate.
For example:p1 = 234 barg,p2 = 1 barg,temperature = 20 ° C,
Density @ T1, p1 = 202 kg/m3,molecular weight = 23KJ/Kmol,Cp / Cv = 2.31
compressibility = 0.59
You can calculate the Cv if you know the mass flow rate, or you can calculate the mass flow rate if you know the Cv.  One depends upon the other. 
The most fundamental Cv equation: Cv=Q Sqrt(SG/Delta-P)
Q would be flowrate
SG=specific gravity (water=1)
Delta-P= inlet pressure - outlet pressure, PSI. 
This equation does not apply to your choke, but it still gives you the idea.  
Then you would find you lack the most essential of necessary information: the required mass flow per time unit.
This can not be calculated, but has to be determined from other facts.
The pressure drop you give is so large that it is possible you have to use more than one valve in series - I have not looked into this.
If this is a pressure relief valve/situation a single step is possible, perhaps by rupture disc as an alternative.

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