there are 2 ways to do it.
you can try just centering the adjustment screw by counting the turns from end to end and putting it in the middle or just keep adjusting differently until it works as you like.
or
you can have a helper raise the platform. remove a wire from the prop valve solenoid, have your helper command lower and adjust the screw so the platform is just slowly drifting down. (you will have to re-key the truck a few times using this method it will code out 60 series codes because you have the wire off the coil.)

The prop valve is sticking up vertically out of the main hydraulic manifold in the rear left side of the truck, it is the only valve with a cap nut and jam nut you can remove. the set screw sticking up after you remove the cap nut is the adjustment screw.

if your adjustment screw does not seem to change anything the valve is faulty.