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For spray heads you just use the manufacturer's chart.
When you use the radius adjustment on a spray you are simply reducing the water pressure by closing a small valve in the nozzle.
As the pressure drops, so does the radius.
Just look at the manufacturer's chart for the radius you plan to reduce the sprinkler down to. Then read the GPM for that radius!
For example, your designing for 30 PSI. The radius at 30 PSI of the sprinkler you selected is 15 feet with 1.85 GPM according to the manufacturer's chart. But you want the radius to be 14 feet. Looking at the manufacturer's chart you see that the radius of the same sprinkler is 14' at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you reduce the radius to 14' will be 1.65 GPM. That's because when turn the radius adjustment screw to reduce the radius to 14' what you REALLY did was reduce the pressure to 25 PSI!
For rotor heads the GPM stays the same no matter how much you reduce the radius!
That's because reducing the radius on a rotor doesn't change the amount of water coming out of the nozzle.
To change the radius a small screw extends into the stream of water coming out of the nozzle. The tip of the screw deflects the water which "screws it up" (pun intended) so it doesn't go as far. This creates another problem, however, which is that it really messes up the uniformity of the water.
So when you use the radius adjustment on rotors, you tend to get dry spots. This is one reason I strongly suggest that you use a smaller nozzle if possible rather than using the radius adjustment screw on the sprinkler. The other reason is that when you reduce the radius you really should also reduce the GPM of the sprinkler. Otherwise there will be a lot more water under the sprinkler with the reduced radius. Bottom line- use the radius adjustment screw on rotors only when nothing else will work .