Rotate relative to target

I want to rotate the top of my turret actor to point to the player. For this I used the FindLookAtRotation node and only rotate on the Z-Axis. Unfortunately this method only works if the turret is placed on a horizontal ground. If it’s placed on an inclined ground it rotates around the world z-axis (red) and not the actor axis (green).

The easiest way to do this is probably to set the rotation relative but how can I calculate the rotation to my target in relative space?

I tried alot but the turret never look at the player. It always have a random angle offset.