Put them about where you think they should be, maybe 1/2 -3/4 inch behind the CG. Fly the airplane. If it gets light on the lines up high in the circle, move the leadouts forward 1/8 inch and try again. If it is Ok up high, move the leadouts back 1/8 inch and fly again. You will probably end up where you are looking right down the airplane's wing and you can see that the airplane is flying tangent to the circle, rather than yawed in or out.