Some hitch manufacturers say to return the front to the unloaded height while some truck manufacturers state half the difference. Probably somewhere between half and full return is correct. A truck with a heavy diesel engine would not need as much to keep sufficient weight on the front tires. A light weight 6 cylinder gas engine may need the most.
The Ford owners manual says:
Weight-Distributing Hitches
When hooking-up a trailer using a weight-distributing hitch, always use the following procedure:
1. Park the vehicle (without the trailer) on a level surface.
2. Measure the height of the top of the front wheel opening on the
fender, this is H1.
3. Attach the trailer to the vehicle without the weight distributing bars connected.
4. Measure the height of the top of the front wheel opening on the
fender a second time, this is H2.
5. Install and adjust the tension in the weight distributing bars so that the height of the front fender is approximately halfway between H1 and H2.
6. Check that the trailer is level. If not level, adjust the ball height accordingly and repeat Steps 3–6