What should the allowed depletion setting be for 4 month old St. Augustine grass, in sandy soil with 2 inch root depth?
I would leave the allowed depletion setting for now.
A 2 inch root zone depth (RZD) will have you watering very frequently.
If you need to water less frequently I would increase your RZD, or decrease your crop coefficient.
If you are having issues with your zone(s), I’d post some screen shots of your soil moisture charts and I’m sure the community can help dial in your system.