Just to clarify some of the confusion with the approach of using the equation above is the area parameter. If 0.015gpm minute is not being applied uniformly across the area, as it more or less would be in sprinkler-based lawn system, the full area of the zone shouldn’t be used. With drip systems the wetting pattern is far from uniform.

The method that @Linn linked to calculates the PR based on total amount of water to be delivered, and eliminates the use of area in the PR calculation. The downside is that you need to have an idea of how many gallons that you want to deliver, but if you do it’s much easier to understand in my opinion. The calculator spits out an area number, but it’s only used in the software to give you reasonably accurate estimates of how much water is being applied when your zone turns on. In that case the area only matters if you care to follow the Rachio usage estimates. In the method that you were given to calculate PR area matters a lot for your PR, but how do you figure out the area of the wetting patterns above ?