Moving the fairlead forward will increase tension on the leech and stop it from fluttering .... but this may not be the answer you are looking for.
First, you have to determine if this is a 'shape' problem or a trimming problem. For trimming, the 'test' is to slowly bring the boat up to the wind and begin to 'luff' the boat (the luff begins to shake). Then notice if the top section of the LUFF (forward edge) OR the bottom section of the luff begins to luff first. If the top section of the luff 'breaks' first then move the fairlead car FORWARD and conversely if the bottom/lower section 'breaks first then move the fairlead car aft .... you watch the LUFF and NOT the leech. Keep testing and adjusting until the ENTIRE luff 'breaks' **all at the same time** when you slowly luff up to wind. This adjustment wherein the LUFF 'breaks' ENTIRELY the same will insure that the 'shape' is good and *matches* the wind conditions that you happen to be sailing in at the time. Different wind-strengths and different sailing angles will change this shape so when in doubt simply head up slowly and watch for which section of the LUFF 'breaks' first, etc. .... you want the LUFF to break simultaneously all along its entire length.
Once you have determined that the shape (as above) is correct, then notice if the leech is still fluttering. If the basic shape is correct and the sail is leech fluttering: 1. increase the leech cord tension (if you have a leech cord). 2. ease the genoa **halyard** tension until the fluttering stops, 3. move the fairlead car 'just a wee bit further forward'.
All these 'adjustments' are made when you have 'proper' backstay (forestay) tension. An overly 'tight' forestay will automatically 'reshape' the genoa/jib leech (exit) section making it 'tripped' (loosened) and such will have a tendancy to leech-flutter (corrected by leech cord tension adjustment).
Its really BETTER to do this with tell-tales applied to the leech and near the luff of the sails as the tell tales are VERY sensitive to correct ariflow across the sails. For a comprehensive (technical) go to www.arvelgentry.com
---> 'magazine articles" ---> then look at:
"Checking Trim on the Wind, November" 1973
"Achieving Proper Balance, December 1973
"Sailing to Windward, January 1974
"Are You at Optimum Trim?", March 1974
These articles by Arvel Gentry are the 'seminal' sail trim (etc.) articles that set the sailing world on its collective ear in the early 70s.
... (also includes the aerodynamically correct explanation of 'how sails really work' - VERY/deeply 'technical'), and sets as total HOGWASH most of the 'so called sail theories' that you find in 'sailing books' and (still) out the mouths of USA highschool science teachers ---- ALL ENTIRELY WRONG.
Hope this is clear and hope this helps.