Hi Travis! Cool project - I like the idea to have the DMA water loss dashboard displayed on a TV in your office.
You mentioned that you "assigned the master/zone meter a negative value". I'm not sure if you are doing this manually when you input the new data each month, or if you are using some sort of expression for your calculations. It sounds like what you want to do would be possible using Arcade expressions to calculate new fields in your feature layer. Then you could treat Meter 2's value as either a positive or a negative depending on the DMA you are calculating.
If you are not familiar with calculating new feature service fields using Arcade, see this blog post: Create Custom Field Calculations Using Arcade Expressions
When your co-workers upload the new .csv consumption values, you could just have them create a new field(s) for the month and run your saved Arcade expressions to calculate the fields before they run the analysis.
PS: If your data is local, for instance in ArcGIS Enterprise, you could probably create a custom Python tool to run all of these calculations locally, and just have your federated feature service published to the DMA Dashboard to display the results. That may be simpler for your co-workers to follow - all the steps being contained in a single custom tool.