The paper presents the methodology for testing Common Rail fuel injectors, which involved extending standard test procedures with additional measurement points and introducing a linear regression coefficient as a new diagnostic parameter. This allowed for a more precise checking of the fuel dosage method, as the active experiment was conducted at a wider range of nozzle opening times (actuation). The obtained results were used in the calculation stage, which was transferred to the popular spreadsheet environment. This eliminates the need to modify the test bench software. The proposed solution should be particularly useful in problematic situations in which a clear assessment of the technical condition of the fuel injector remains difficult, as shown in the example.