Hey @Kontur , sorry for the late response. Apparently they need to approve my first few posts on the forums.
I am also using UE 4.18 and the “get relative rotation” works for me. I made 2 videos for you to see how that works for me. The first one is my blueprint setup, in which you can see how I use that relative rotation to turn my character if the 3rd person spring arm is exceeding my 134 degrees threshold to the left or right of the character.
In the second video, you’re able to see the actual output of that rotation of the 3rd person spring arm relative to my character capsule, whatever it’s world rotation is. 
https://www.youtube.com/watch?v=AdvcnDlhoSw
Hope this helps you in your endeavour.