How to get the final pawn rotator after set an offset with the camera boom?

Hi all,

I am working with a 3rd shooter game, now I set the cameraboom with an offset (0, 50, 0).
how can I get the correct fire rotator after this?
If I use the ControlRotation, it looks always have an offset with my crosshair(in the center of my screen).

thank you for any help.

Now I use GetControlRotation.RotateVector(Offset)

its worded.