Hello DaleJEckart,
Unmentioned previously is that the cables should not have any elasticity. They should not be able to stretch in length.
The differential motion is obtained by having the cables at the servo attached about 5 to 7 degrees ahead of the servo axis of rotation; Closer to the front of the model.
Tension has already been explained.
Zor
The Kevlar apparently does not stretch... not quite sure about differential motion...