Great discussion here, and thanks to Franz for his "dog food" diary.
IMO, the current Flex Daily implementation is close to spot on. For the Iro to work best, we need to dial in both the watering rate and the usage rate. The problem with the slider idea is it affects only one of the variables and as soon as weather conditions change the user has to keep tweaking it (or overwater and never realize it).
It's been some work (we have a 28-zone system), but it's been worth it. Getting vegetation, slope, sun and soil type is easy, and every Iro user should take the time to configure each zone. Measuring actual watering rate (and setting up custom heads when needed) is more time consuming, but once you do it you have it forever. With those in hand, I believe the only thing one really needs to change is the crop coefficient to "dial in" their yard.
The basic problem with the slider (and the existing +/- adjuster in Durations) is that they essentially trick the Iro into thinking more/less water is being put down than actual. This becomes a problem when adding actual water from rainfall, as the sums get screwed up. This is why getting actual watering rates is useful -- then the amount of water being put down is always correct in the system.
So, if the water is correct, "dialing in" means getting the usage adjusted. In my view, starting with the standard vegetation settings is the way to go. Measuring actual root depth seems to be overkill. In a mixed zone (i.e., shrubs under trees), pick the primary vegetation type with the shallowest depth. And in a newly planted zone, try adjusting the standard by reducing the root depth until it becomes established. From there, if a zone appears underwatered, adjusting the crop coefficient will complete the dialing in procedure.
Because the Iro recalculates the moisture levels every time you make a change to a variable (a great feature, BTW), one way to dial in the crop coefficient is to make small adjustments, save them, look at the moisture levels again and push the water balance to zero on the day before stress appeared in the plants/lawn. Basically, this approach says "since the water input is correct, the water must be being used up more quickly than the current settings indicate".
This approach has worked great for me, and seems to be working as the temperature swings in NJ from the 60s to the 90s. Still doing a little tweaking when I see a stressed zone, but the changes are becoming smaller and less frequent. And once I have all the zones to the point where they are never underwatered, I'll probably start making some crop coefficient adjustments (and maybe test greater water depletion) in the other direction to make sure I'm not overwatering anywhere.