Valve sizing: choosing the differential pressure

Nanimo

Hello,

I'm trying to find the correct solenoid valve for my application (I want to be able to shut down the flow of gas from a distance). Tried to find which Cv correspond to my application following some control valve sizing procedure I found on the internet. (I'm totally new to this thing).

My problem is: a differential pressure drop across the valve is always asked and I can't figure out what to choose for that.
I'm using a pressure regulator and a mass flow controller on my system. The solenoid valve would be between the pressure regulator and the mass flow controller. The differential pressure I chose to size the valve is the one that might occur if the pipe would fail after the valve (i.e. the pressure of the pressure regulator, lets say 72 psi, minus the atmospheric pressure).
It then gives me a Cv of 2.6 for my application. Which seems reasonable, but I have no idea if the way I've chosen my pressure difference is the correct way to do it.