OpenProj - Project Management

Hi,

I'm looking for specific advice for working with OpenProj. I have to note that I never really used a project management software to its fullpower before, please pardon my ignorance.

My problem is that I can't figure out how to track tasks. Let's say that I have a task with a duration of fourteen days. After two days someone sets completion to 50 percent but leaves duration and end date untouched. After another day - oh wonder - the task is finished. How do I do this correctly so the Gantt chart reflects "saved time" correctly? If I just set end date to the current day duration is off (it's set to seven days), end date seems to be recalculated from and completion is set to 100 %. It seems that the tool first tries to fill up completion and then calculates duration and end date from it. Any other changes to end date won't stay. So how do I use the tool correctly to enter finished tasks?