Provided are methods and fuel injection systems implemented with a plurality of injectors coupled with a common rail, the common rail coupled with a pressure sensor, and the pressure sensor coupled with a processor. The method includes: identifying, by the processor, one of the injectors to calculate a pressure change rate of the common rail associated therewith; receiving, by the processor, pressure measurements of the common rail from the pressure sensor before and during an injection event within a measurement window; using, by the processor, a pre-injection mean pressure of the common rail to determine a rail pressure drop range that is specific to the identified injector; and calculating, by the processor, the pressure change rate associated with the identified injector based on the pressure measurements of the common rail taken during the rail pressure drop range.