I have similar soil that goes 80 feet down. My kids call it mouse powder. The presets in the app aren't going to be right. Assuming you are in the USA, go to the USDA's Web Soil Survey (WSS). Hit the big green WSS button. Zoom to your location then select your "Area of Interest" its in the toolbar far right. I suggest you select at least a ten acre patch so you can see if there are different soils in your area.
When the AOI is set, it will be cross-hatched in the map view. Now hit the second tab in from left, labelled Soil Map.
There should be a Key on the left with a list of soil type in your area. Find your type and click it.
That will pop up a description of your soil. In the description scroll down until you find the Properties and qualities heading. You want to find the Available water storage line. In my case it was the bottom of that section.
You take that inches number and divide it by 12 to get the Available Water(in) number for your zone setting (ie 4.3 / 12 => about 0.37). That number is actually a PERCENTAGE and Rachio should fix that label.
The preset for sand showed 0.05 which seems insane. The preset popups do nothing in my zones since the first save. I have to enter the numbers.
The next thing you're going to want to figure out is your Crop Coefficient which most soil publications call (Kc). Your state university agriculture department may have publications with those numbers. In my case I first had to find the mixture of grass seeds used locally via the ag dept publications. Then I searched for Kc numbers for Kentucky bluegrass and red fescue. These numbers vary by the month so you may want to average the months of your hot/dry season. In the end, I got a final average of 94%. That number is plugged into the zone as Crop Coefficient of 0.94. If you need help with that match I setup a spreadsheet I could share.
I hope this helps you. It took me about 6 hours to surf my way to this info. I'm testing it in 2 of my 8 zones right now.