Hi @ngnam1988. I do not think that will be possible exactly the way you want it. We do not have direct control over that angle. All we specify is the 2D points on the sheet for where to place the tip of the arrow, and where to place the text. The true physical size of the text itself and the true physical size of the flat landing of the leader under that text are automated by whatever styles you are currently using, and even that is not super exact, but they all are included in where the top/outer end of the angles portion of that leader will be, in relation to where you specify the text location to be. You can review the following online help pages for more information about those Inventor API objects.
LeaderNote
LeaderNote.Leader
Leader
LeaderNotes.Add
Wesley Crihfield
(Not an Autodesk Employee)