Sizing an Orifice Plate for a specific Pressure

Ok folks... I currently have a centrifugal pump flowing a flow rate Q1 through a pipe of diameter D1 at a pressure of P1. The pressure is created by orifices restricting the flow downstream.

I want to add an orifice plate of diameter D2 in order to reduce downstream pressure to P2.

So what I'm asking: How do I specify the diameter D2 to obtain a specific P2?

My specifics:
Q1 = 125 gal/min
D1 = 2.635 in (inside of a 2-1/2 Sch 10 pipe)
P1 = 35 psi

I want P2 to be approximately 20 psi. Doesn't need to be exact. I'm not concerned by how much Q2 decreases.

Thanks!