What reference volume (estimated or actual) do you use to measure drinking water losses?
Are you referring to real losses (associated with physical leaks in the piping system) or to apparent losses (associated with theft and faulty meters)?
The IWA and AWWA have put together a free audit software tool to help utilities calculate the volumes and costs associated with water losses:
This document is lengthier but the executive summary is worth the read:
Hope this helps!