I guess the way to understand our logic, is that the spray radius shows what the head is actually capable of, not how closely you would like to place them.
In this case, they show the RVAN1724 as being 23' (24), and able to be reduced down to 17' (hence being called the 1724).
If you add the head to the project at 80% of its radius (with the latest update), then it is as you state, a maximum of 18.4', and able to be reduced down to the same minimum, 17'.
That said, obviously you have more experience with this head, especially in a windy situation such as Hawaii. To us it seems outlandish to spec a 24' rotary for a 13' area. So if you could elaborate a bit more, on some of your knowledge of how the head operates and how you have found it to work in real-world conditions, that certainly helps us a lot.
The primary "problem" we were solving with this last update, was that we had users mention that a head added at 80% radius was able to be reduced to well under what it actually threw at, so the update was all about matching the actual products more accurately.
--J