@RVRoadie. Assuming that you used the Orbit cups and their calculator, you can use the precipitation rate from their calculator and then, yes, plug it into a custom nozzle. Just make sure that you enter the value for in/hr. You mentioned GPM in your posting so I wanted to be sure.
You are right. I get the PR(in/hr) from the Orbit calculator, and also the Distribution Uniformity. I create a custom nozzle with the PR rate, and use the Distribution Uniformity to set the Efficiency %. I fix up the area to match the size of the station, and leave everything else alone.
Thanks @franz. FYI a couple of us determined a while back that the hydrorain and orbit calculators are identical. Catch Cups. @RVRoadie ended up getting to it from the Orbit side.