I assume you're using 8.0 for scaling here?
_y *= 8.0;
_x *= 8.0;
The scaling needs to take account the current horizontal FOV and viewport/screen width. I've found that if your FOV is 90 degrees using (ViewWidth/2) as the scaling factor works well.
If it's not 90 degrees FOV, you can approximate it via
90.0/fov * (ViewWidth/2)