wagas, the book says 5.5" back from the LE at the wing root for starters. They say you can go back to 6.5" or more. Mine is at six and a brunette, and is still a bit nose-heavy. And I have two servos mounted where that miserable bell-crank assembly was.

Be warned that the alieron horns are screwed into the soft LE wood. Both of mine pulled loose and almost fluttered the plane to death at 35 flights. The wood is so chewed up now that I'm going to have to cut the entire section out and inset a piece of hardwood. If you catch it before the wood fails, you can inlay 1/64 ply scabs on both sides for the horn and screw plate to bear against.