How to set curve point that will determine the extrusion line

 From:  Metin Seven (SEVENSHEAVEN) 3 Dec 2013  (5 of 19) 
6327.5 
Merci encore, Pilou. :)

Here's an illustration of my question:



Do I have to trim the curve before extruding, or is there an easier way to determine the point that will become the extrusion line?

EDITED: 3 Dec 2013 by SEVENSHEAVEN