So 100% full really means 50% of the Max. Allowed Depletion (MAD).
FULL (MAD=50%) = 0.2 (in water)/(in soil) * 6 in soil * 0.50 = 0.6 in water
It can't, and it would indicate roll-off. I don't know the specifics of why 120% was chosen, but I know that there's some amount of inaccuracy of what 100% is in the app. versus the reality of what's happening in your lawn. I don't know if that has anything to do with it, but I'm OK with it due to that reason.
I guess it's a matter of opinion to a point. For me, if below 50% means I'm below the permanent wilting point, I don't plan on seeing many data points there. I always like to see as much of my data on the screen as possible, so 50-100% works well for me.
I'm not sure I'm following. How would that be different from what's under the adjust button ?