Complementing what WillRussell said, that is the best way to forecast your end date. The way to do it is:
- Enter the advance per activity.
- Make sure you tell MSProject the status date of the project by going to Menu>Project>Project Information>Status Date (here you enter the status date)
- Then go to Tools>Tracking>Update Project
- Check the radio button "Reschedule uncompleted work to start" (put the status date if other)
- Press OK.
Project will reschedule uncompleted work (your remaining 20%), i.e. 2 days in your example, to start from the status date on.
Note: if after doing this, it is not showing an accurate en date as it will happen, then you have to modify the duration of your activity.