The angle of the lead out from the bellcrank determines the angle that the nose of the aircraft addresses the arc of the flight circle.  In others words, shifting the lead outs aft on the wing will cause the nose of the model to turn outward causing the plane to be tighter on the lines, it will also cause the plane to slow down.  This is often used when there is a bit of wing to keep the lines tight. 
     The norm is for the aft line to be straight out from the bellcrank and the front line to angle back, moving the front line will alter the models angle.

Larry

Not for any particular model but a half A. What effect would having the leadouts too far forward or too far aft cause ?
And what would the easiest way to find a starting location for them be ?
I'm using an adjustable lead out but not sure where exaclty to lock it down for the first flight and there is no drawing...

Jim,
If you start with leadouts too far back the model will yaw until you dial it in.
If you start with leadouts too far forward it will probably cause slack lines ending in a crash.
Adjustable is good because you may find slight differences when using different length or type of lines. That said, most of the time the position shown on plans is usually pretty close and very few of my 1/2A planes have adjustable leadouts.

George

But this is a wing I built and designed and a fuselage i cut by hand to match and fly.....
Basically too back is better than too far forward I am guessing. And in the example of not having enough room to go a lot further forward that is,  OK I am guessing.