The subject of a future video
Never having flowed the orifice, the best you can do is get close.
Formula (solved for pressure for ya):
PSI = ((GPM / (CV * AREA ))² * SG)
The "2" is supposed to be a "square" (smallish, upper two-thing)
CV is the kicker. If you have the known, flowed value for GPM at any SG and any pressure, you can figure the CV for the orifice. Then you can change the formula around to use the CV and some new parameters and get the new pressure or flow. Since you presumably don't have a flow number to use to get your real CV, use 21 for an Enderle bypass pill and 23 for an Enderle nozzle jet. The CV differs based on the size and shape of the hole...also the individual characteristics of each hole...the surface finish, leading chamfer, exit edge, length of the orifice.
Also, the above is what is used to figure the pressure on the leading side of an orifice to get the GPM through it of the fluid of known SG. You might have to do some other figuring to get "pressure drop" in your system.
Spud