Original/Remaining Duration

Hello,

    I am using Primavera P6 for project management. I have a situation which I see as a problem for myself. If someone explains this to me, it will be really better for my management.

    The situation is that; I am entering the the start and finish date for an activity. ( The activity is started so I am checking the started box). Then I am putting a "finish on" constrain to the activity. (The same date as finish date). After that, I am scheduling and I see "negative total float". I want my activity to finish on date which I assigned as "finish on date". However, the program is taking the finish date for a forward date and it shows me negative float. 

     I see that the activity has the same original and remaining date, that's why I have the negative total float. Because my scheduling time is after the start date. But I want to make the constraint and finish the activity on finish date.

    Any help?