Sorry for the post but after 3 hours of browsing and searching the FAA pages i found the answer for my question:
(1) The destination runway length shall be determined prior to takeoff to be at least
115 percent of the runway field length required by the provisions of 14 CFR part 121,
§ 121.195(b).
The calculation shall be made prior takeoff with the actual GrossW and there should be a 15% margin in the calculated length and the available length