Dear Brooks,
Developing a drop down window on the toolbar is very hard to implement. As for right click on the task to change the completion percent from context menu, there are two possible variants that can be developed:
The first one is a "change completion" option with drop down menu with the percents of completion (for example: 5%, !0%, !5% 20% etc.).
The second one is "increase to 10%" option.
Which variant do you like more?