Here is my exact process from Water Use It Wisely. The steps below correspond to the same steps in the website.
Step 1: I picked the following average canopy sizes for my larger shrubs and trees, and found the appropriate gallon recommendation
Step 2: Take note of my GPH per shrub and tree as directed by the site. In my case it was one, 1 GPH emitter per shrub, and three 2 GPH emitters per tree. Here’s what they state for step 2.
Step 3a: Use the information from steps #1 & #2 for the entries in the calculator they have in this step. Hit ‘Calculate’, and see the recommended run time.
Step 3b: Hit ‘Next’. They list a table for Phoenix that suggests watering frequency and root depth. I used these to approximate a root depth. Since I had decent values in place for AWC, and with Rachio’s magic, I automatically got the intervals they suggest. Check!
Step 4: I went into the Rachio settings for nozzles, and adjusted the PR down until I got to the suggested 8 hr times for shrubs, and 4.5 hours for trees.
Done.
Coudn’t Step 4 be easily spit out by a Rachio calculator once it has the information from step 3 so I didn’t have to make the empirical adjustment to PR? As you well noted, there are comments in the Water it Wisely site about moving the heads out to the canopy circumference and adding heads as the vegetation grows. In my case I haven’t had to add heads beyond the three per tree. Perhaps I got lucky because of the development stage of my vegetation. Our yards also tend to be reasonably dense in terms of the trees/shrubs, so I also think there’s some amount of H2O stealing from one plant to another for those that have longer roots. Don’t tell my neighbor, but I’m pretty sure my large Ficus is drinking up his water too. It’s not ideal, but if Rachio had this type of calculator built into their site I think it would get people a lot closer to a decent starting point, and would minimize the frustration highlighted by this thread. A single-point-emitter Rachio setup calculator like this could (and probably should) have a bunch of disclaimers on it that state the recommended uniformity and placement of emitters. That way people would know that this is an approximation, and the further they deviate from the assumptions the worse their estimate will be.