Dear John.Depending on your ...

Depending on your project task, and final result requirements there are different ways of calculating and evaluating water losses. Working with water losses last 10 years I can recommend you to start calculating water losses based on IWA blue pages where you can find some basic guidelines in water losses analysis. There are many good papers online for this subject on which you can refer to but try to stay in IWA methodology domain for first calculations.

Basically your work will consist in several steps:

1. GIS of water network

2. Hydraulic model

3. DMA zoning

4. Field measurements

5. Hydraulic model calibration for preforming further analysis and determine average pressure per DMA 

6. Calculation of performance indicators (CARL, UARL, ILI, RL/m, RL/mH2O, RL/HC, etc.)

Those indicators will give you starting point in decision making and defining further actions. You can avoid calibrated hydraulic model but then you need some other method of defining average pressure per DMA.

If you have some other questions, feel free to contact me.

Best regards...