How do I calculate the clearence in a needle valve for a pressure drop

[SalvadorT]

I am using a precision needle valve to achieve superheated steam. I would like to drop the pressure from 100+ psi to 20 psi, but I do not know what clearence I need to allow at the throat to allow this pressure drop? Does anyone know an equation to solve this?

 

[hacksaw]

the manufacturer of you needle valve performs flow testing for such valves and will provide such a calculation or a flow chart of some kind. The calculation is not done from first principles because of the complexity of the flow path.