The top retainer should have a flat side, and the shock rod a corresponding flat.
so when you have it apart, you can hold the retainer to tighten the nut to a certain point. Then a slight tweak in the car to make sure it's tight.
It doesn't not need to be done up super tight.
I can do it up tight enough by holding the retainer whilset it's out of the car, but always check to see if I can get a bit more when it's inthe car. If the spring is trying to turn, it's tight enough.