Convert world location to screen location in third person view game to get socket location?

Hi, guys.

In my third person view game, I want my trace my character’s head position by draw rectangle on it.

So I attached socket to the bone of head. And in my level blueprint I get the world position of the head socket, converted the world position to screen location using ‘ConvertWorldLocationToScreenLocation’ node, devide the screen location by viewport size to scale it 0-1.
Then in my post processing material, I made custom node that compares the screen position(0-1 scale) of the current pixel with the scaled screen position of head socket. If the screen position of head socket is close to the screen position of current pixel, this post process material changes the color of the current pixel to blue.
But the result is strange. blue rectangle is shown near the character’s waist, not head. I checked if I positioned the socket to the right place, and I did no wrong.
How can I solve this problem?

Are you just trying to make an outline? You could just add a fresnel to the emmisive color to do that