Is there some DIY method of figuring out the percentage in the soil? In other words, if Rachio says there is 75% moisture, is there a way for me to go to the zone and check, and not wait for signs of stress?
Yes, you could break it down into some calculation I’m sure, but that is @rraisley domain 
…but there are a bunch of components to it, including, MAD (Maximum Allowed Depletion, or Allowed Depletion in Rachio), Available Water Holding Capacity (Available Water in Rachio, derived from soil type). This chart is actually a pretty good visual of what Rachio moisture is following. Our 0-100% scale is from red line (50% MAD/AD) to 100% Field Capacity.
In this next graph, you can see how your Available Water (which translates to Field Capacity) varies by soil type.
These charts are from the following University of Minnesota Extension, and many schools have some GREAT information from their Ag schools.
I was actually thinking something more practical. Like, if I dig a hole down X inches, and the soil feels damp/wet/dry, can I translate that into some idea of how much water is actually available? I have 2 zones that are currently scheduled with some very long breaks between watering (10 days and 8 days) and I’d like to be able to check at say day 5 to see if they actually are still good, until I get used to this new way of watering.
How about an inexpensive soil moisture meter like this https://www.amazon.com/dp/B014MJ8J2U/?tag=xponics-20
I guess you could…but I personally am lazy I guess and would look for signs of stress…
Like @Alturia said, you could buy an inexpensive soil probe, and use the chart I posted to keep your grass in that AW range…but you also need to keep in mind that if you previously watered very frequently, for short duration, your grass has no reason to go deep with its roots. It might take some time for the plants to adjust and start going deeper with its root structure, and the only way to monitor that is to watch for stress.
I have found the inexpensive moisture meters to be worthless for lawn moisture. Even bought a $100 one, and it’s not really better. One fellow who’s posted here, I think it’s for a golf course or other very large expensive grassy area, gets graphs from a company that shows lines with percent moisture across the who property. I was drooling. But only in the budget for millionaires, I think, for private properties.
I’ve found that sticking a long, think screwdriver into my clay soil can be a good indication of how deep the water is. But remember, we’re trying to keep the lower half of the root depth /constantly/ damp, while varying the top half from zero to 100% saturation. That really complicates any “real” calculation. That’s why Racho does the calculations it does, to help us in this area.
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Neither zone is grass. One is shrubs (sage) and a tree, in a raised planter/swale, so the area is rather small, but I am reasonably sure that the roots are deep. So that one I am just looking to be reassured as I get used to the longer gaps between watering them. The issue with watching that one for stress is that the tree is a Carolina cherry, so a non-pine evergreen, and some of the leaves are turning yellow. However, it is spring, which is when it puts out new leaves, so that could be normal.
The other zone is mixed flowers (cannas, jasmine, asparagus fern, lavender, Mexican heather, and a lone camellia), on a drip system, in heavy shade (except the camellia which is more like half shaded). The issue I am running into there is that the ground looks dry, but if I dig down an inch or two, it feels cool and moist (not wet). The plants look to be doing ok, except the camellia which is showing some bronzing on the leaves, which I’m thinking may be more an issue of too much light and not a lack of water. It’s an on-going issue that I have had with it. But the ground that is visible between the plants looks so dry at the surface!
So, just looking for reassurance while I get used to it.
I don’t see a moisture probe doing anything for the deep rooted trees. Most of the water is going to hang out below the depth most meters can reach. Might be fine for the mixed flowers as they probably don’t dig much below 12".
My wife has a “Butterfly Garden” with annuals, perennials and such, and is in my back yard zone which is grass. My grass wants watered twice a week, but the flowers can’t take that in the heat. So I have the zone on a Flex Daily schedule, but have a separate Fixed schedule that waters the zone for 2 minutes in the afternoon, to cool things down and add some moisture. This helps the flowers and doesn’t really affect the grass. The Flex Daily program actually cuts back on the normal watering a bit, because it knows I applied the 2 minutes a day.
Maybe something like that would help with your Camellia.
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