Systems and methods for determining the fraction of a first phase of a multiphase fluid flow stream, such as the amount of water in crude petroleum oil flowing from a production well or container. An electrical property, such as permittivity, and a physical property, such as density, are used as the basis of the improved characterization. The method is particularly well-suited to reduce salinity-dependent uncertainties for wells experiencing high water cuts. A time series of measurements is collected, and the extrema of the observed values are used to generate a hindsight auto-calibration and correction to the other values using knowledge of the degree of uncertainty in the measurements caused by variable salinity and variable phase state of the multiphase fluid. The hindsight auto-calibration and corrections thus permit more accurate measurements of the instantaneous and the cumulative amounts of each phase in the multiphase fluid flow stream.