You are multiplying your location by 18.000. Not your direction. The direction is a normalized vector with the size of 1 unit and represents the path the trace should take. You only add the result of that multiplication with your location.
Also the more commonly used approach here unless you need to be able to click on locations on the screen is to use your camera to get those values.
“Reference to camera” → “Get World Location” → Trace Start
“Reference to camera” → “Get Forward Vector” multiply by your number (e.g. 18.000) and add your world location → Trace End.
Since your camera is where you’re looking at it will always be the exact middle of the screen.
Cheers