Today I came across software on CNET that will size orifice plates for given design conditions, compute pressure drop for a given flow, or flow for a given pressure drop. This software is designed for gas or liquid orifices. You can download the ISO-5167 2003 6.5 application from CNET and evaluate it for yourself.
Download ISO standards, ISO 5167 for measurement of fluid flow or AGA Standards from ANSI.
There's no connection between ANSI, ISO or AGA and NorCraft Software (this is not an endorsement of the software or the company).