Actually, you know what would be handy as well? A progress wheel. As in, not only could you type in a number for the percent complete, but then provide an on screen slider or wheel that you can turn to incrementally move the progress forwards or backwards. And even show the progress on the screen. i.e. show the map and the planned path and highlight or color change the line to show the requested “progress” and move the highlight/color as you turn the wheel so you can hit where you want it to start with some precision.