For as long as I can remember, I've used the easy 1 mm = 1 L = 1% soil volumetric water content (VWC) relationship when converting between water use by grass, ET, irrigation, and soil VWC. Bill Kreuser shared the simple calculation to work this out in inches, for any rootzone depth. The post discussed here is https://www.asianturfgrass.com..../post/soil-water-con more about all kinds of turfgrass topics at https://www.asianturfgrass.com/
Find turfgrass information and decision-making tools at https://www.paceturf.org/
See the PACE Turf YouTube channel at https://www.youtube.com/user/paceturf
See the ATC newsletters at https://subscribepage.com/atc_newsletters
Listen to the ATC Doublecut with Micah Woods podcast at https://atc-doublecut.transistor.fm/
Listen to the ATC Office Hours podcast at https://atc-office-hours.transistor.fm/