I could work out the problem for you, but the best thing for you to do is break out your Crane Technical Paper 410 and go through the process. If you don't have one, you should get one.
The easier thing to do is to download software off the web. I would recommend FE-Sizer. It is a great flow element sizing program that works great for sizing orifices or in your case determining the flow across an orifice.
Good Luck