When you say 12" deep lawn, are you referring to the soil depth or root depth? Grass roots will be less than all of the other landscape plants.
And you have a lot of different plant types including grass. All of those root depths are different. You will have to fine tune that zone until the water duration works for the environment. Or just make it a bed zone without considering grass. It would seem that shade from the landscape plants would eventually prevent healthy grass growth.
Increasing and decreasing root depth will effect the zone run times.