It’s not exactly the mouse location you use in this type of situation. Either because the mouse is locked at center of screen or you would stop being able to rotate once the mouse reached screen edges.
What you want is Mouse Delta, ie. how much has the mouse moved between frames. Go to Project Settings>Input and add two Axis inputs. One will be for horizontal delta and the other for vertical. Then add as input for each of them Mouse X and Mouse Y, respectively.
In your PlayerController right click and type the names you gave to your Axis inputs. It’ll create Events that you can get the Axis scale from to control the head rotation, adding some amount of degrees to it each frame. These events will be constantly fired (each frame) so you don’t need to set their values to variables and use on Tick. When you’re not moving the mouse, they give you 0. When moving, they give you how many pixels were the mouse moved between frames, positive or negative depending on direction moved.
These three videos from Unreal teach a lot in this subject: