Do you have any experience or method of calculating the oxygen demand of a DWC growbed?

To answer your question, I have found calculating the OD of plants difficult. From a biological standpoint, plants have a very low OD; the microbes use most of the OD in the DWC. Due to environmental changes, the exact levels may change throughout the year making measuring the BOD and COD on a commercial farm impractical. When maintaining an airflow of ~16 LPM/ m3 of water, I have maintained DO between >6 mg/l with any crop up to ~35C. Many factors can affect this, so experiment in your climate before scaling.