Convert world rotation to relative

In my 2D game I have a character at the middle of the screen. My blueprint character have Flipbook component inside. And the component have component “Body” as a child. Also the Body have Arm as a child. And Arm have weapon. All components have their own rotation.

Then, when the game is going, I tap on the screen. I got rotation from hero location to the touch location. And I need arm to point to the touch location. But if I do “Arm component → Set Wold (or relative) rotation”, then I got something like this:

How to set rotation for arm (or any child-component) properly?