For that rate, the manufacturer (RainBird in this case) is assuming 100% overlap of sprinkler throw radius (i.e., 4 sprinklers in a square, positioned exactly 15 feet apart for the HE-VAN 15s). As noted, I don't use them that way for that particular zone, having only one row of sprinklers on one side of the strip, giving about 0.75 in/hr. Moreover, with experimentation, I found that, at 30 psi, the HE-VAN 15s throw about 12 feet, the HE-VAN 12s throw about 10 feet, etc. So I spaced the 15s about 12 feet apart, and double-checked the overall flow rate to the zone to verify the precipitation rate. So, anyway, it's easy to see that entering a custom nozzle as "HE-VAN 15" and assigning it a (zone) prec. rate of 0.75 in/hr if just confusing, since the zone prec. rate is not a simple function of the nozzle type.
Most of my zones are like that, requiring entry of a misleading custom nozzle type to define what should be a directly entered prec. rate (or flow rate and area). For example, one of the zones is connected to an above-ground pattern sprinkler (an impact sprinkler which allows adjustment of throw distance based on angle - very cool). The water flow to this sprinkler must be adjusted to ensure that the throw range stays within the irregularly-shaped zone. And by measuring the water flow and area of the zone, I can calculate a prec. rate. However, having to enter that as a custom nozzle type is nonsensical. There is no typical prec. rate for such a sprinkler, it depending mainly on adjusted flow rate. And I should be able to enter flow rate and area directly, without the need to create a custom nozzle type, or to have to precalculate the prec. rate.
Another example is a zone where I'm using Hunter 9SST nozzles, throwing 9' x 18', at 1"/hr without overlap. iow, these nozzles are designed to water a strip without needing to have their throw overlapped, meaning you need fewer nozzles per zone (compared to HE-VANs, for example). So they're still in category of "fixed spray head", but operate differently with respect to positioning and number. This case comes closest to where a custom nozzle type "9SST" corresponds to a well-defined prec. rate, but that's not because its in the category of "fixed spray head", but rather because the nozzle (with my fixed pressure spray head) has been specifically designed to provide a consistent prec. rate without overlap.
So, anyway, you can see why I'm skeptical of specifying custom nozzles as an indirect way of entering prec. rate. And for anyone less experienced, picking a nozzle type as a substitute for knowing actual flow or prec. rate probably just leads them to corrupt most of what Rachio is trying to do wrt water management. imho