For the two types of grass listed, Cool season grass and Warm season grass, the default root depths are 6" & 9" with a Crop Coefficient of 80% & 65% respectfully. As I was taught, not all Cool Season grasses are the same, just as not all Warm Season grasses are the same. So, it is best to do some research on these (and some other variable) to get them correct. Obviously, Rachio had to come up with some defaults and I am sure they did not just chose random numbers, but went off of some research. Maybe the focused on certain grasses that are more drought tolerant and typically have longer roots. I do not know the various types of grasses, just know what I have according to the sod company. Maybe more exists that Rachio could add in the future, I do not know. I think @twin1 and I are just saying that suggesting the measurement of the grass length to get an estimate of root length might work in some cases, but not all especially trying to encourage drought tolerant lawns with the types of grass that we have. I am definitely not trying to start a debate, but maybe miscommunicating my thoughts. I apologize if I sounded that way.
With my lawn as mentioned, the holes I have done to do things with sprinklers have consistently shown my roots as long or longer than the square ended shovel (11") that I am digging the holes with. Incidentally, I do have a corer, but so far the shovel is easier to see since I have needed to dig anyway.
Just like the different grass types, we can say the same thing about the nozzle rate, soil types, and maybe more. With the default values, it is best to do some research or testing to see if they are accurate or need adjustment to our reality. My nozzle rate defaults to 0.7, but two types of measurements/documentation show 0.5. My soil type seems to be Loamy Sand which defaults the available water to 0.07, but a reputable website calls it Sandy Loam (0.12), but available water at 0.06.