Ongoing Tasks in P6

What is the best way to set up an activity that I want to run throughout the length of a project?

For example, I have an activity in my plan called "Updates to Risk Register"

This starts on 17-August-2011 and finishes on 20-Sep-2012

I have ticked the Started box in the Status tab.

When I run a schedule with an updated data date, I want the progress % complete to calculate based on the end date of the task, and not for me to have to work out how much remaining duration there is?

Should I set the % Complete Type to Physical, Duration or Units?

Any other advice on setting up these kinds of tasks in P6?