I’ve plugged it into my spreadsheet so I can play with things and see what happens – Here’s the calculation the best I can tell.
You will need this:
Scheduling Multiplier = 1/(0.4+(0.6*efficiency))
How much to water in inches = Available water inches * Root depth inches * Allowed Depletion
Duration = (60*how much to water in inches)/(in/hr)*Scheduling multiplier
Yeah … I’m sometimes that nerdy to want to know this. My calculations seem to come up with the same amount as Rachio comes up with.