Our Rain Bird irrigation controller had this. The Orbit B-Hyve irrigation controller the Rachio 3 I just installed replaces has it: The ability to tweak the water budget for an entire schedule up or down by a percentage.
So, for example, during on-going wetter-than-normal weather I could reduce all the zones in a schedule to, say, 80% of normal. During drier-than-average weather I could tweak the entire schedule up to 120% of normal.
I understand the philosophy of how Ranchio means their controllers to work: Let weather forecasts, weather, and Rachio’s algorithms handle it. And I may get there, someday. But not today 
So, in the meantime, it would be nice if this feature existed.
Otherwise, with my current manual schedule, I’ll have to tweak each of the eight zones up or down, individually. Not handy.
Alternatively, or perhaps additionally, the option to have Flex Daily type adjustments made to an entire manual schedule?