Trying to have My body Rotation follow Players head look using Rinterp HELP!

Hi Im working on a vr project and created a custom ue4 mannequin skeleton.
Instead of having the pelvis connected to the root, I connected the spine 3 and neck bone to the root, Which Is located where the head bone is.
This was done for the needs of my VR setup and is working very well.
The head & neck are using an IK setup , working fine also…

What I am trying to achieve is to have the body YAW rotation follow the head YAW rotation, But not 1 to 1.
Which is what its doing right now, I want to use R interp to make it start rotating when The head yaw has reached a certain threshold.

I have made head look stuff. and another VR project I can switch control of head and body rotation on the fly.
So I have an understanding of what I am doing with Rinterp for the most part .
But I have spent all day trying to crack this & every possible combination I can think of. But can get it to work.
Its either 1:1 or having a freaking out.
And advice would really help me out and same me some sanity