regulating co2 with a pressure regulator

I'd like to set up co2 augmentation, I already have a tank with a regulator but the regulator was for welding so it is a pressure regulator instead of a flow regulator, my question is given the diameter of the hose and the pressure, does anyone know how to calculate the CFM i would be putting out or the equation that i would need?.

thanks.

according to the co2 calculator my 20lb tank should LAST 175 hours@ 1CFH, any way to calculate how long a tank should take to discharge at a certain psi or is it hit and miss?