How do I size a metering disk to limit flow pressure?
Well, for a start, in the case of pipe diameter anywhere from 100 to 250 mm, the theory says your opening should be between 30 and 35 mm. You may have to do a bit of trial and error if you are targeting a specific flow rate.
Here’s a calculator: http://www.tlv.com/global/US/calculator/water-flow-rate-through-orifice.html
Let me know how it turns out.