So, I have never put mine in on an actual build it is not actually needed. I am slowly working on updating the instructions now and I will add the cross clamp screw part soon.
When you put it in you need to make sure your Leadscrew is not moving. Meaning it is directly above the Z coupler. There is a measurement test and a movement test. the top of the leadscrew should stay the same distance from the outside face of the YZ plate when the gantry is all the way up, vs all the way down. Much easier on a LR4 than an LR3