Transients in the rate of injection (ROI) with respect to time are ever-present in direct-injection engines, even with common-rail fueling. The shape of the injection ramp-up and ramp-down affects spray penetration and mixing, particularly with multiple-injection schedules currently in practice. Ultimately, the accuracy of CFD model predictions used to optimize the combustion process depends upon the accuracy of the ROI utilized as fuel input boundary conditions. But experimental difficulties in the measurement of ROI, as well as real-world affects that change the ROI from the bench to the engine, add uncertainty that may be mistaken for weaknesses in spray modeling instead of errors in boundary conditions. In this work we use detailed, time-resolved measurements of penetration at the Spray A conditions of the Engine Combustion Network to rigorously guide the necessary ROI shape required to match penetration in jet models that allow variable rate of injection. The discrete control-volume jet model of Musculus and Kattke is utilized, and improved to account for variable spreading angle with axial distance, also based upon experimental measurements. Considerations are also made for effects such as gas initially in the sac of the injector, as well as variable fuel temperature at the start of injection. The penetration-derived ROI shape is in agreement with an “educated” ROI, developed considering hydraulic models of the injector as well as known experimental problems with ROI instrumentation. In the end, improved ROI shapes are made available for detailed CFD modeling of engine spray and combustion. The methodology provides advances beyond traditional analytical model analysis, which lacks the capability to treat variable ROI or spreading angle.