I see different solutions for calculating run times etc. I finally worked out a way to do this that I hope works.
You need two elements, size of area to be watered and flow rate. Area can be stepped off and I use a flume water monitor for flow rate.

An example;
Area to be watered: 480 sq ft
Flow rate: 15 gpm

480x1/12=40 cubic ft
40x7.5=300 gallons to provide 1 inch
300/15=20 minutes run time
60/20=3.0 nozzle inches per hour

Easier and more accurate than filling up cups.

Good starting point. Obviously uneven watering will need changes in efficiency etc.

Yes, I have 26 valves, so I have 3 units. I Have a flow meter on the first unit, so it knows flow rate for each connected valve. So just entering the area to be watered is enough information for Rachio to do these calculations and generate watering time and nozzle inches per hour.
Units 2 and 3 are downstream from unit one, but unfortunately flow data is only available to the unit itâ€™s directly connected. Donâ€™t want to add two more flow meters.

Not JBTexas but you have to measure the area for each zone. You also have to know the flow rate of your sprinklers in each zone (total flow in gpm when the zone runs). Then use the calc above. If you want to know where the formula comes from then check here: How do I calculate my precipitation rate?

Note that when you tell Rachio the brand of nozzles that you have, it will default to specific precipitation rates. If you have Hunter MP Rotators, it defaults to 0.8 in/hr. Most MP Rotators are actually 0.4 in/hr. Anyway, if your system is designed for head to head coverage, your precipitation rate will be greater than the default sprinkler head value because the area will be watered by at least 2 heads if not more depending on system design. This is one reason why the 2 values need to be determined and a value calculated.

The formula is the standard imperial calculation of precipitation rate. I donâ€™t have the metric equivalent, but something like:

Precipitation rate (cm/hr) = constant x flow (liters/hr) / area (sq meters)

Itâ€™s available in any practical textbook on flow calculations, shown online in forums that discuss crops, growing things, etc. Here in Texas, the university extension services give classes in topics like this, and the USDA has online calculators that help you grow a lawn.

Rachio does do this calculation for you if you have a flow meter that can provide that flow value, and you can measure your lawn. Turns out that precision irrigation (which is what Rachio is designed to do) requires information about whatâ€™s being irrigated. Itâ€™s important to be precise if you plan to apply ONLY the water necessary to keep your vegetation alive and healthy. That will be, for most conscientious Rachio users, about a quarter of what you spent before. That ainâ€™t chicken feed in Texas, so Rachioâ€™s saved me many multiples of the cost of the product and my time worrying about it.

As we lawn geeks say it: â€śFlex daily and forget about it!â€ť

Iâ€™ll add further about all those calculations including flow rates of nozzles, catch cups, efficiency settings, etc. That, in my humble opinion, is not that useful. Thatâ€™s why people are astounded that how much irrigation water going on their lawn can be determined from just two numbers - the meter reading and the size of the lawn. Believe it, itâ€™s that simple. I know you can do it.

â€śNozzle inches per hourâ€ť is Rachio-speak for Precipitation Rate. If youâ€™ve figured out (from your water meter, a flowmeter calibration, or any other method) the flow value, and you know the size of the field, just calculate the Precipitation Rate and plug it into Nozzle Inches per Hour. (This is, of course, if youâ€™re using Flex Daily scheduling, which can make use of it.) If you have a flow meter, Rachio still uses the Nozzle inches per hour value while it monitors flow and leak detection. Flow meters for small spaces are flaky and subject to wide pressure fluctuations based on time-of-day, pump (if you have one) efficiency, your neighborsâ€™ watering practices, pressurization (pipe filling) characteristics, settle time (pipe emptying) characteristics and other vagaries. It think thatâ€™s why Rachio exposes the NIPH (new acronym I just made up - pronounced â€śniffâ€ť) so we can all tweak it as we see fit.