I believe a zone can only have one flex daily, so if your flex daily schedule already includes all zones, then yeah, cannot add a second. You can add multiple flex daily for different zones though.
My Rachio has 8 zones on 2 schedules, on different sides of my property. Both schedules are Flex Daily, one (north) runs Tu,Thu, Sat - the other (south) Wed, Fri, Sun. Monday is for mowing and maintenance.
I’ve never, in over 3 years, encountered a situation that those schedules failed to sufficiently provide enough water. That includes all seasons and all temperatures in northeast Texas. When we were briefly rationed to 4 hours on 3 days per week, we moved all the zones from one schedule (which we disabled) to the other and experimented with “begin at a time” to see if we would exceed 4 hours. We never did. When the restrictions were lifted, we restored the temporarily moved zones and re-enabled the disabled schedule.
You can have multiple flex daily schedules, but you can’t configure a zone to appear in more than one. This may seem restrictive, but if you can deliver enough water, it won’t be. (Should note that we run a tight razor’s-edge watering schedule for warm season grass. Stress is our grass’s best friend.)
There is a bug n the adjusting water duration that it doesn’t show future water amount when you adjust the duration. It only shows up in the water journal after the fact.
That is great to have a reminder of how that works. It might be nice to have the ability to reset it to the original recommended/calculated time. But then again, maybe that is not needed with the new feature.