It's a matter of trial and error.
The aim is to get the model to fly tangent exactly to the circle otherwise, if for example the lead out is too far aft, you'll get the model to fly with the nose out of the circle and pulling the lines very much, but loosing tension overhead, and vice versa, if the lead outs are too far forward, you loose tension in normal flight.

Also, theoretically at least, even if you have found an optimal position and are comfortable, if you change the CG position, and/or line thickness, you will need to change lead out position.

If I remember corectly (haven't flown these some 30 years or so), you start with a lead out position about 2 deg. aft of your CG and check during test flights.
BTW, the bellcrank position is not important here if I'm not mistaken, only CG and lead out positions affect the line tension.