Best & easiest way to capture your data would be with a GPS or other bicycle computer that can download data to your computer. Starting from a dead stop and with the device turned-on and recording, you will get downloadable data that can be plotted and overlaid from the multiple runs for your comparative analysis. It will be well below scientific grade, but it eliminates human error with regard to stop watch operation, the need for an assistant or the need to take your eyes off the road to watch your computer.