As I remember there is a stop on the lift shaft to set the up limit but there is nothing to set the down spot and I have never seen a reason for it. When your using an implement you want to drop it all the way to the ground and release all the pressure on it so it will float on the ground with the weight of the attachment doing the work. There is no power down on your tractor as there is on some tractors so the implement will rise and fall with the contour of the ground.
If you want to stop it at some point let it down to that point and release the lever. It may not stay there because the cylinder will tend to leak off after a while.
Why do you want to stop it at a certain spot?