The simplest method to determine the size of an orifice plate is to use BS 5306 : Part 2 : 1990 Appendices B page 149 (This table is also published in FOC 29th Edition).

This sets out a formula and tables to allow you to determine the orifice K factor and the bore size.

When you specify an orifice plate in FHC the plate is located at the begin node. So if you have Node 300 to 310 the orifice plate would be at node 300. If you require the orifice plate to be a different location then you will have to insert a node number at that point in the systems first.